The second presidential debate was formally canceled late Friday after President Trump refused to participate in a virtual event, the latest sign of the upheaval that the coronavirus has wrought on the 2020 campaign.
The Commission on Presidential Debates had tried to shift the debate to a remote format, given Mr. Trump’s illness and the uncertainty about his health, but the president soundly rejected the proposal and instead planned to resume his signature rallies, beginning on Monday in Florida.
Instead of the debate on Oct. 15 in Miami, the nation may instead be treated to dueling forums on rival networks.
Once Mr. Trump pulled out of the debate, his Democratic challenger, Joseph R. Biden Jr., scheduled his own town-hall-style event that night on ABC, leaving the commission little choice but to abandon its plans. Mr. Trump’s campaign was in talks on Friday with NBC to hold a counter-event for the president, most likely on the same night, according to three people with knowledge of the discussions.
“In seven decades of televised presidential debates, this is the first debate to be canceled,” said Newton N. Minow, a commission member who helped to create modern televised debates in 1960. “The loser is the American voter.”
One final debate now remains scheduled, on Oct. 22 at Belmont University in Nashville, with Kristen Welker of NBC as the moderator.
Behind in the polls, the Trump campaign has pushed to add another debate to the calendar with or without the debate commission’s involvement. “We would be glad to debate one-on-one without the commission’s interference,” the campaign said in a statement Friday night.
The Biden campaign has so far rejected the notion of an additional date, saying Mr. Trump should not be allowed to modify the debate calendar to his liking.
The continued jockeying over the debate calendar came as Mr. Biden finished up a Western swing in Las Vegas, completing one of his busiest weeks on the road as the president remained confined to the White House because of his illness. On Friday, Mr. Biden lashed out at Mr. Trump for his behavior in the last week since he tested positive for the virus, saying the president is willing to ignore experts and put his political interests above the health and safety of others.
“His reckless personal conduct since his diagnosis, the destabilizing effect it’s having on our government, is unconscionable,” Mr. Biden said. “The longer Donald Trump is president, the more reckless he gets.” He delivered his remarks wearing a mask before a drive-in audience in Las Vegas, as supporters honked in approval from their cars.
Later, Mr. Biden, the former vice president, dismissed Mr. Trump’s plans to renew in-person rallies. “Good luck,” he told reporters. “I wouldn’t show up unless you had a mask and were distanced.”
Earlier on Friday, Dr. Anthony S. Fauci, the country’s leading infectious disease expert, said on CBS Radio that there had been “a super-spreader event in the White House,” referring to the Rose Garden event where many mask-less attendees celebrated Judge Amy Coney Barrett’s nomination to the Supreme Court.
The sparring over the debates seemed likely to drag on.
The Biden campaign has said it would participate in the Oct. 22 debate regardless of the format, although the Democrat’s aides would prefer a town-hall-style event, as the canceled Miami debate was to have been. The Trump campaign is also onboard with the date, but angry at the commission for changing the Oct. 15 event to a virtual form.
In a statement on Friday, Tim Murtaugh, Mr. Trump’s communications director, proposed a debate on Oct. 29 — a date the Biden campaign had already declined — and added, “There’s nothing that says that President Trump and Joe Biden can’t debate together without the overlords at the commission having a say in the matter.”
The Biden campaign, for its part, called it “shameful that Donald Trump ducked the only debate in which the voters get to ask the questions.”
Aides to Mr. Trump reached out to multiple networks to explore the possibility of a competing town hall after Mr. Biden announced his ABC event.
The NBC town hall with Mr. Trump will likely occur only if certain safety conditions are met, including Mr. Trump testing negative for the coronavirus, according to two people familiar with the conversations. A network moderator, production crew and undecided voters would all be situated near the president at the venue.
NBC hosted an outdoor town-hall-style event this week with Mr. Biden, and the network had extended a similar invitation to Mr. Trump — although that request was made before the president fell ill.
As the debate over the debates raged, the Biden campaign pressed its advantage on the airwaves, surpassing the $500 million mark in paid digital and television ads this year, compared with $438 million for the Trump campaign, according to data from Advertising Analytics.
On Friday, Biden campaign officials rolled out seven new political ads, including one that is narrated by the actor Samuel L. Jackson and is aimed at mobilizing Black voters, and a rare ad designed to mobilize Asian-American voters.
On the trail on Friday, Mr. Biden warned Democrats of the need to “win overwhelmingly” in November to stop any “phony challenges” about the integrity of the election.
“We can’t just win,” Mr. Biden told Latino leaders in Las Vegas. “We have to win overwhelmingly.”
Annie Karni and Nick Corasaniti contributed reporting.