Debates are often unpredictable, but it is especially hard to game out how this debate featuring a moderate standard-bearer and a liberal challenger will unfold and how people will process it. Hundreds of thousands of viewers, if not millions, will have been personally affected by Sunday, as public gathering spaces are shuttered, schools are closed and on Thursday the stock market plunged by the largest percentage in decades (it snapped back upward on Friday).

“It is going to be a different kind of debate,” Senator Chris Coons of Delaware, an early ally and surrogate for Mr. Biden, said with more than a little understatement.

The extraordinary situation offers an unusual challenge and opportunity. It will provide a national platform and audience for Mr. Biden and Mr. Sanders to preview their own brands of presidential leadership, especially in contrast to that of President Trump, who had spent weeks downplaying the threat that the coronavirus presented.

“You have to throw the entire playbook out the window and focus on what people care about because the agenda has shifted dramatically since they were last onstage together,” said Jared Leopold, a Democratic communications strategist who worked for Gov. Jay Inslee of Washington during his 2020 presidential bid. “It’s like you were preparing to pitch in the World Series and suddenly you found yourself at bat at a key moment. The dynamics have totally shifted.”

Stephanie Cutter, a Democratic strategist who worked on the Obama campaigns, said moments like this are when “voters want to see leaders, not politicians,” warning against anything that would look like “petty politics.”