Biden’s Shaky Debate Performance Has Democrats Panicking

That judgment extended beyond the political class. Mr. Biden’s perceived odds of winning the nomination plummeted within hours on PredictIt.org, a betting site that takes wagers on political events. His chances of being the party’s candidate fell to 60 cents, down 26 cents, meaning that bettors essentially thought there was only a 60 percent chance of him being nominated even though he swept the primaries, has no internal opponent and controls the party apparatus.

No incumbent president has dropped out of the race so late in the campaign cycle, and there was little consensus about what would happen if he were to. On Thursday night, Democrats were imagining scenarios in which party elders like Senator Chuck Schumer of New York, former Speaker Nancy Pelosi of California and Representative James E. Clyburn of South Carolina were to intervene with Mr. Biden.

There was no indication that any of them would agree to do so. Other Democrats said they feared it was too late, noting that Mr. Biden is a proud, stubborn man who has long insisted he is the best equipped to defeat Mr. Trump and would not listen to anyone other than perhaps his wife, Jill Biden, who has strongly supported another run. Democrats have long fretted that there is no obvious successor, uncertain that Ms. Harris, Mr. Newsom or any other party figure could rise to the challenge.

Incumbent presidents often stumble in their first debate of a general-election season, either because they are rusty or overconfident, but in many cases make up for it with stronger subsequent performances. Mr. Biden’s troubles particularly brought to mind Ronald Reagan’s first debate in 1984, when he appeared old and out of it; he salvaged his campaign at his next debate with a well-timed joke about not exploiting his opponent’s youth and inexperience.

The problem for Mr. Biden is no other debate is scheduled until Sept. 10, meaning he has no obvious opportunity to recover for months. And as the veteran Democratic strategist said, this was not like Mr. Obama losing to Mitt Romney in 2012, which was a tactical setback. “This is existential,” the strategist said.

So, rather than resetting the campaign in Mr. Biden’s favor, as it had anticipated, the president’s team ended the evening knowing that the task of the next few days if not weeks would be to stem the damage and rally the party behind their beleaguered leader.

Zolan Kanno-Youngs, Katie Rogers and Annie Karni contributed reporting.