Mr. Biden, who entered the Wisconsin primary in a dominant position to claim the Democratic nomination, has staked his bid on presenting Mr. Trump as both a historical anomaly and a reversible blip, a president whose removal alone would restore some sense of national equilibrium, in the candidate’s telling.

“I believe history will look back on four years of this president and all he embraces as an aberrant moment in time,” Mr. Biden said in his announcement video last year. “But if we give Donald Trump eight years in the White House, he will forever and fundamentally alter the character of this nation.”

Mr. Biden has never shown the work on this math, never explained how the threat could be both existential and quickly revocable.

But with the task of social and economic repair now awaiting whoever takes the oath in nine months, two things can be true at once: A return to normal sounds terrific to many voters. And there is plainly no normal to return to, if there ever was.

“The striking thing about the first term is how much damage he was able to inflict,” said Robert Reich, a former labor secretary under Bill Clinton who endorsed Mr. Sanders in the primary. “At the margin, he probably could do more with two terms, and I wouldn’t wish that on this nation. But he’s already done a huge amount.”

Those with experience in revert-to-normal campaigns likewise caution against any wishful thinking.

Tim Pawlenty, the former Republican governor of Minnesota — who took office in 2003 after Jesse Ventura, the retired professional wrestler — suggested that politics was “a lagging reflection of our culture,” where “massive demographic, economic and technological change” in recent years has often steered voters in both parties toward populist messengers.

“The pandemic will change our culture temporarily, but it won’t change what’s been fueling our longer-term political trajectory,” he said. Mr. Pawlenty, who has said he voted for Mr. Trump but called him “unhinged” in 2016, predicted that a mix of populism and “our culture’s blending of politics and entertainment will likely yield more ‘larger-than-life’ candidates being elected.”