President Biden is cruising to the Democratic nomination. Former President Donald J. Trump could begin to wrap up his party’s nod within days.

America’s response: This can’t be real.

Even as both men stroll toward likely summer coronations and a fall rematch, an undercurrent of disbelief is coursing through the country. Many Republicans view Mr. Biden as so politically and physically weak that they think his party will replace him. Many Democrats can’t fathom that Mr. Trump could win another nomination while he is facing 91 felony counts and four criminal trials.

This incredulity — ranging from casual doubtfulness to conspiratorial denial — has lurked beneath a year of polling showing a deeply gloomy public mood, and has emerged in dozens of interviews over the past two weeks as well as recent declarations from candidates and political commentators.

“They’ll pull a switcheroo at the last minute,” David Lage, a Republican missionary from Spring Hill, Iowa, said of Democrats. “They’ve tried about every other dirty trick.”