A day after former Vice President Joseph R. Biden Jr. accepted the Democratic presidential nomination with an emotional speech in which he pledged to restore American norms and “overcome this season of darkness in America,” President Trump countered Friday that the Democratic convention had been the “darkest and angriest and gloomiest” in history.

The back-and-forth over darkness and light was a vivid illustration of the near-Manichean terms in which the two parties have cast this year’s election.

Democrats, led by former President Barack Obama, warned urgently at their convention that the future of American democracy was at stake, while Mr. Trump spent the week painting an apocalyptic image of what will happen if he loses.

“I’m the only thing standing between the American dream and total anarchy, madness and chaos, and that’s what it is,” Mr. Trump told a conservative group, the Council for National Policy, on Friday. He said he represented “the bright future, not the dark future that you saw last night and for the last four days.”

Mr. Biden made a case for his vision of a post-Trump America in his convention speech on Thursday night, the most important in his career.

“Here and now I give you my word: If you entrust me with the presidency, I will draw on the best of us, not the worst.” Mr. Biden said. “I will be an ally of the light, not the darkness. It’s time for us, for we the people, to come together. And make no mistake: United, we can and will overcome this season of darkness in America.”

He never mentioned Mr. Trump’s name. But the former vice president quickly made an appeal to voters who aren’t predisposed to support him, saying that he would try to win them over as president. He condemned bigotry. He yoked the Trump administration’s failures in its coronavirus response to the economic crisis and the financial fragility that millions of people are facing, months into the pandemic, with no end in sight.

In remarks Friday, Mr. Trump continued his assault on the mail-in voting that many people plan to use during the pandemic. He suggested sending law-enforcement to monitor polling stations, and shared “a theory” that if the election results are not known by the end of the year, House Speaker Nancy Pelosi, a Democrat from California, would become president. (He appeared to be referring to Section One of the Constitution, which says the House of Representatives picks a president if electors do not vote by the inauguration.)

“Crazy Nancy Pelosi would become president, you know that,” Mr. Trump said. “No. No. I don’t know if it’s a theory or a fact, but I said, ‘That’s not good. That’s not good.’ ”

Now, with the Democrats wrapping up a convention that was forced to go remote by the coronavirus pandemic, in which speaker after speaker made the case that Mr. Biden could heal the nation after the Trump era, Republicans are preparing to make their own case for giving Mr. Trump four more years when they gather at their own convention next week.

Louis DeJoy, the embattled postmaster general, defended his first two months overseeing the Postal Service to a Senate committee on Friday and said he was “extremely highly confident” that even mail-in ballots sent close to Election Day would be delivered on time.

“We will scour every plant each night leading up to Election Day,” Mr. DeJoy said as he testified for over two hours before the Senate Homeland Security and Governmental Affairs Committee.

Mr. DeJoy, whose cost-cutting and operational changes have prompted widespread concern about mail-in voting, called suggestions that he might intentionally slow ballot delivery to help President Trump “outrageous.”

“There has been no changes to any policies with regard to election mail,” Mr. DeJoy told the lawmakers, adding, “The Postal Service is fully capable and committed to delivering the nation’s election mail fully and on time.”

Under questioning from Democrats, however, he refused to unwind other steps, like removing hundreds of blue mailboxes and mail sorting machines, that he said were initiated by his predecessors in response to a steady decline in mail volume. He denied knowing about the machine removal when it first began, telling senators it was “not a critical issue within the Postal Service.”

Mr. DeJoy, a major donor to the Trump campaign who took office in May, is at the center of a firestorm as recent cost-cutting measures at the Postal Service — including reducing overtime and removal of mail-sorting machines — have led to delivery delays and fueled concerns that the post office might not be able to handle a crush of mail-in ballots for the 2020 election in the midst of a pandemic.

Mr. Trump has stoked those concerns by repeatedly questioning the legitimacy of mail-in voting, which he baselessly claims is riddled with fraud. “The honorable thing to do is drop the Mail-In Scam before it is too late!” he wrote on Twitter last week.

Even as Mr. DeJoy testified on Friday, six state attorneys general sued him and the Postal Service in federal court, arguing that changes at the service were an attempt to disenfranchise voters.

Though Mr. DeJoy said earlier this week he would suspend some changes until after the election, Josh Shapiro, the Pennsylvania attorney general, said in a statement announcing the suit that Mr. DeJoy had not addressed some of the suit’s core concerns. He called for “detailed information” on how the agency would reverse what he said were “illegal changes.”— Emily Cochrane, Nicholas Fandos and Matt Stevens

With the 2020 Democratic National Convention now in the history books, here are seven takeaways from the gathering:

• Joseph R. Biden Jr. delivered the performance he needed. Mr. Biden is not his party’s smoothest or most electrifying speaker, and President Trump and other Republicans have spent months mocking his record of verbal missteps (despite the president’s own long record of falsehoods and gaffes). But on Thursday, Mr. Biden delivered his sharpest, most powerful address of the campaign — and benefited from the low bar his opponents had set.

• Kamala Harris broke barriers. Ms. Harris, a California senator and former state attorney general, made history Wednesday when she accepted the vice-presidential nomination, making her the first Black woman and first person of Indian descent to be nominated to a national ticket of a major party. Her prominent presence throughout the convention underscored the generational and racial diversity she brings to the ticket, two factors in her ability to energize Democrats as Mr. Biden’s running mate.

• Former President Barack Obama’s speech surprised, and revealed him as the moral conscience of the party. Mr. Obama delivered an unusually passionate, caustic attack on his successor in the White House and described the stakes of the election in stark and chilling terms: the continuation of American democracy as we know it.

• Mr. Biden won the nomination — and for now, the messaging battle in his party. This week it was clear that even Mr. Biden’s ideological opposites within the party — including Senator Bernie Sanders, the avatar of the progressive movement — had embraced Mr. Biden’s case for building the biggest possible coalition to beat Mr. Trump, policy differences aside.

• Jill Biden helped the campaign emphasize an urgent issue for many Americans: education. Dr. Biden, an English professor by trade, reintroduced herself as an educator who is intimately familiar with the grave fears many American parents are experiencing in this pandemic moment.

• Democrats have papered over their policy differences, at least for now. Democrats hold sharply divergent views on policy questions including how to expand health insurance coverage, how to regulate Wall Street and Big Tech, and how much to value bipartisanship. But this week saw little policy debate beyond a broad embrace of matters such as combating climate change and gun violence, welcoming immigrants and improving access to health care.

• Democrats extended a hand to independent voters and Republicans. Were those voters listening? Even as Democrats spent the week moving to unify the party around Mr. Biden and Ms. Harris, the convention plainly prioritized outreach to moderate voters who may not identify as Democrats but are disillusioned by Mr. Trump’s stewardship of the Republican Party. The critical test for Mr. Biden coming out of the convention will be: How many undecided voters heard and warmed to that message?

How Convention Speaking Times Reveal Democrats’ Pecking Order

In the run-up to the four-day virtual event, speakers jockeyed for more air time. Here’s who had the most.

Joseph R. Biden Jr.’s campaign and its joint committees raised more than $70 million during the four-day Democratic National Convention, the campaign announced on Friday.

The campaign’s fund-raising blitz continued Friday afternoon at a virtual event that featured Mr. Biden and his running mate, Senator Kamala Harris, along with their spouses.

During the event, Mr. Biden spoke of the need to urgently address what he sees as President Trump’s failures in office, calling out the president’s handling of the coronavirus pandemic and painting a grim portrait of the future if he’s re-elected in November.

“Things don’t have to be this bad and they never had to be this bad to begin with,” Mr. Biden said during the fund-raiser. “If this president gets four more years, you know as well as I do it’s only going to get worse. Is this what we want for our families, for our children?”

Mr. Biden struck a similar note in an interview he and Ms. Harris did with ABC News on Friday, saying he would shut down the country as president to stop the spread of the virus if scientists recommended doing so.

“I would shut it down; I would listen to the scientists,” Mr. Biden said. “I will be prepared to do whatever it takes to save lives because we cannot get the country moving until we control the virus.”

During the interview Ms. Harris also responded after Mr. Trump called her “extraordinarily nasty” last week, using “nasty” or some version of the word no fewer than four times to describe her.

Ms. Harris largely brushed off his comments in the interview, saying they were “designed to distract the American people from what he is doing every day.”

During the fund-raiser, Mr. Biden, Ms. Harris and their spouses spoke highly of one another, outlining the origins of their relationships and how they have gotten stronger since. Mr. Biden’s wife, Jill Biden, praised Ms. Harris’ role as a “pioneer” for women of color, while Ms. Harris spoke about the growing bond between her and Dr. Biden.

“Doug and I have been talking about our friendship, and what it means to he and I to have and be with you and Joe,” Ms. Harris said of her and her husband, Douglas Emhoff.

Mr. Emhoff recalled Mr. Biden campaigning for Ms. Harris during her Senate run in 2016, and how Mr. Biden swore her in the following year, saying he would “never forget it. Mr. Biden also poked fun at the new title Mr. Emhoff, who would become the first-ever second gentleman, would carry if the ticket wins in November.

“Doug will break ground as an incredible second gentleman,” Mr. Biden said. “You’re just a plain gentleman, pal.”

The Democratic National Convention attracted an average nightly television audience of 21.6 million viewers, a respectable number given the many Americans who have turned away from traditional TV sets in favor of online video and livestreams. The Thursday broadcast, when Joseph R. Biden Jr. accepted the nomination from Wilmington, Del., drew the convention’s biggest crowd, 24.6 million Americans.

Overall though, live TV viewership fell 18 percent from the 2016 D.N.C., according to Nielsen.

Nielsen statistics do not include streaming views, which are difficult to credibly measure.

MSNBC, home to liberal favorites like Rachel Maddow, had the highest-rated prime-time week in its 24-year history. Its coverage ranked ahead of every other TV network on every night of the convention.

Shortly before Mr. Biden spoke on Thursday, President Trump called into “Hannity” on Fox News for a live interview. About 4.6 million people watched Mr. Trump’s appearance; the Fox News audience then dropped by 36 percent when the network’s 10 p.m. convention coverage began.

As the Democrats’ first fully remote, virus-era convention wrapped up, our chief television critic, James Poniewozik, assessed the strange spectacle. While “incendiary applause lines don’t work the same when no one is there to clap,” he wrote, the format does work for “gravity and heart,” which Joseph R. Biden Jr. emphasized in his speech.

Mr. Biden had concrete criticisms and counterarguments to the president, especially on fighting the coronavirus: “It didn’t have to be this bad.”

But it was when Mr. Biden talked about feeling the pandemic and its devastation that you could feel the speech take off.

He asked to speak to those who had lost someone to Covid. He had lost too, he said — the convention had reminded us of this, and a four-night character arc paid off in this moment. “I know how mean, cruel and unfair life can be sometimes,” he said. “Your loved one may have left this Earth, but they’ll never leave your heart.”

There have been over 170,000 Americans lost to the pandemic but no real public mourning. Mr. Biden didn’t say anything about the president who, visiting the site of a mass shooting last year, posed with an orphaned baby and gave a thumbs up. He didn’t say that the country might miss having a leader who believes that feeling loss doesn’t make you a loser. He didn’t need to.

The camera pushed in closer. Mr. Biden, like his running mate, Kamala Harris, spoke from a podium in a near-empty hall, which gave Ms. Harris’s speech a haunting air on Wednesday. This time, the camera held him tight in the frame, matching his intimate speech. It wasn’t written as if it were meant to rouse a cheering crowd in the room. It was written to the camera, like a presidential address, and reached through the screen to the other side.

The speech did end like Ms. Harris’s, with music anticlimactically playing in the desolate hall as Mr. Biden waved to supporters on a big screen — a substitution much pandemic-era TV has made for live audiences that will never stop feeling uncanny.

Facebook spent years preparing to ward off any tampering on its site ahead of November’s presidential election. Now the social network is getting ready in case President Trump interferes once the vote is over.

Employees at the company are laying out contingency plans and walking through postelection scenarios that include attempts by Mr. Trump or his campaign to use the platform to delegitimize the results, people with knowledge of Facebook’s plans said.

Facebook is preparing steps to take should Mr. Trump wrongly claim on the site that he won another four-year term, said the people, who spoke on the condition of anonymity. Facebook is also working through how it might act if Mr. Trump tries to invalidate the results by declaring that the Postal Service lost mail-in ballots or that other groups meddled with the vote, the people said.

Mark Zuckerberg, Facebook’s chief executive, and some of his lieutenants have started holding daily meetings about minimizing how the platform can be used to dispute the election, the people said. They have discussed a “kill switch” to shut off political advertising after Election Day since the ads, which Facebook does not police for truthfulness, could be used to spread misinformation, the people said.

We spoke with Ronna McDaniel, the chairwoman of the Republican National Committee, about her party’s national convention, which starts Monday. (This conversation has been edited and condensed.)

Tell me a little bit about what Americans should expect from the Republican side next week.

You’re going to see a lot more of real people and a policy-driven discussion about the difference that the Trump administration has made in people’s lives and the things that they’ve done during the past four years, and then a preview of what the president is going to do when he’s re-elected.

So, what’s the second-term agenda?

You’re going to see a tribute to America as we talk about the greatness of this country, a tribute to our history, our heroes, and it will be very aspirational.

Is there a central message Republicans want voters to take away from next week?

Democrats are featuring Hollywood celebrities who play real people, and the Republican convention will be about real people. We don’t need to hire actors to play real people. The Trump administration has always been about average everyday working Americans and their story. We don’t need screenwriters. We don’t need fiction. We’ll talk about real America and celebrate that.

There have been plenty of real people featured on the Democratic stage, though.

There’s not a lot of interaction. It’s all Zoom.

A notable number of Republicans spoke on the Democratic stage. What do you make of that?

Former Republicans.

What about John Kasich, the former Ohio governor who ran for the Republican nomination in 2016?

Kasich has classic Trump derangement syndrome. He’s not liked this president since the minute he was the nominee, even to the degree of not even coming onstage in his own state when it hosted the convention in 2016.

It’s sad to watch them be part of a convention and a platform that is, in fact, ushering in socialism and fundamentally transforming the United States of America, as we know it.

So you’re saying that if you are speaking at a Democratic convention, you’re not really Republican anymore?

That’s what I think.

 

Democrats are planning a broad slate of programming next week to coincide with the Republican National Convention and counter the shift of attention to President Trump.

In an effort to paint Mr. Trump as an agent of “chaos,” the Democratic National Committee is expected to run TV and digital ads nationally and in battleground states that it said highlight “the many ways Trump has failed as president.”

The Democrats also plan to host virtual events with Americans who have been affected negatively by the Trump presidency, and will circulate information in direct response to events at the Republicans’ convention.

“We’re very interested in what will happen in their convention,” said Tom Perez, the Democratic National Committee chairman. “We’ll take advantage of the haphazard nature of their convention to continue to take our message to the American people.”

During the Democratic convention this week, President Trump hit the campaign trail, holding rally-like events in swing states including Minnesota, Wisconsin and near Joseph R. Biden Jr.’s hometown in Pennsylvania, and tweeting attacks during the convention itself.

It was a stark departure from previous election cycles, when presidential candidates have traditionally gone dark during the other party’s convention.

Mr. Biden’s campaign has not said whether he or his running mate, Senator Kamala Harris of California, would themselves hold events next week.

The Democrats plans to assign themes, such as “Economy in Crisis” and “Health Care in Crisis,” to each day of the Republican convention.

Democratic leaders will host calls with the news media on each day of the Republican convention. Representative Val Demings of Florida will host on Monday; Gov. Gretchen Whitmer of Michigan and Senator Cory Booker of New Jersey on Tuesday; Speaker Nancy Pelosi on Wednesday; and the former Democratic presidential candidate Pete Buttigieg on Thursday.

 

A new drive to get Kanye West on the ballot in Arizona is underway after similar efforts in the swing states of Ohio and Wisconsin were rejected this week.

A signature-gathering campaign in Arizona is being led by Edee Baggett, who is only the latest Trump supporter to aid Mr. West. In April, she expressed support for the president’s re-election on her Facebook page: “He is my President … he is our President and he should be everyone’s president as he is working for us,” she wrote, while allowing that “he’s crude and can be rude” and “should tweet less.”

In an interview, Ms. Baggett, who runs a signature-gathering firm called National Ballot Access, stressed that she worked on both sides of the aisle — for “probably more Democrats than Republicans.” She said she had done work for the campaigns of Tulsi Gabbard and Marianne Williamson, adding that “nobody that I know from any party has told me I need you to do this on the sneak. I don’t work that way.”

Mr. West was rejected by elections officials in Wisconsin on Thursday, because his signatures were submitted just after the deadline, and on Friday in Ohio because of problems with his nominating petition and statement of candidacy.

Many Democrats view him as a spoiler, since several of the president’s allies and supporters have been linked to his campaign, including a Trump campaign lawyer who delivered Mr. West’s signatures to Wisconsin elections officials. Mr. West also recently met with the president’s son-in-law, Jared Kushner, for what Mr. Kushner described as a “general discussion more about policy.”

Mr. West is still aiming to get on the ballot in Virginia, Idaho, Wyoming, Kentucky and North Dakota, where another signature-gathering group, Let the Voters Decide, is working on his behalf. Signatures must be delivered in Arizona by Sept. 4.

“This is a very short deadline,” said Ms. Baggett, who was hired about a week ago. “All we can do is try.”