To Republicans, Virginia represents the promise of renewal, the chance to rebuild their party in a fairly forbidding state — and without having to make the difficult choice of fully embracing or rejecting Mr. Trump. Addressing supporters near a farmer’s market in Old Town Alexandria Saturday morning, Mr. Youngkin said his victory would send “a shock wave across this country.”

Suffering yet another loss here, though, would make it clear to Republicans that they cannot continue to delay their internal reckoning over the former president and that, even in exile, his unpopularity remains the party’s biggest impediment…

Should Mr. McAuliffe lose or barely win, moderates will demand immediate passage of the infrastructure bill. Liberals will argue that the Democratic Party, and democracy itself, are in such a parlous state that they must push through new voting laws. And strategists across the party’s ideological spectrum will be made to contend with a political playing field in the midterm elections that stretches deeper into blue America.

“We’re going to have to change our calculation of what’s a race and look at the districts Trump lost,” said Rebecca Pearcey, a Democratic consultant. “Even if he wins,” she added, referring to Mr. McAuliffe, “we’re going to have to reassess what the map looks like on Wednesday, because Tuesday is not going to be a pretty night.”