At the same time, Democrats, fretting about President Biden’s sagging standing in public opinion polls after the messy withdrawal from Afghanistan, would generally be thrilled to make what is shaping up to be a challenging midterm election next year a referendum on Mr. Trump. The president’s party almost always loses dozens of seats in the House during the midterms; Democrats picked up 41 seats in 2018 and Republicans flipped 63 in 2010.

For the moment, there are signs that Republicans are more energized across the country. They’re inundating school board meetings to talk about how race is taught in classrooms, and in some parts of the country they’re filling candidate training rooms at a pace not seen since 2009.

A potential third Trump campaign, started more than three years before the next presidential election and 18 months ahead of the next Iowa caucuses, could refocus all of his party’s energy onto himself and away from the right-wing cultural issues other Republicans see as political winners.