Party members are weighing different incentives in their determination of whether to get behind Mr. Trump. Republican members of Congress on the ballot face a particularly delicate task. Especially in the states where Mr. Trump performed well in the primaries, many Republicans believe they can’t distance themselves too much from their nominee because they will need his supporters for their own political survival.

Politicians thinking of a presidential run in 2020 have their own incentives. If they believe that Mr. Trump’s brand of populism will become the new normal in the party, they’ll need to adapt accordingly. But if Mr. Trump loses badly to Mrs. Clinton and his candidacy damages the party, those who failed to oppose him aggressively will be held to account.

Two Republicans to watch are Gov. Scott Walker of Wisconsin and Senator Marco Rubio of Florida. Both are plausible contenders in 2020 and will soon need to decide between urging their supporters to back Mr. Trump or remaining holdouts in hopes of re-creating a more traditional conservative coalition in four years. Mr. Walker, on Wednesday, said he would support Mr. Trump because he’s better than Mrs. Clinton but wouldn’t commit to campaigning for him.