There is, however, some air in the rule — which actually spells out how to fill a vacancy “by reason of death, declination or otherwise” — that an enterprising lawyer could try to exploit. But arguing that Mr. Trump’s nomination can be rescinded by second-guessing party bosses over the word “otherwise” seems highly improbable.

Replacing Him Would Be a Mess

Even if Mr. Trump withdrew as the nominee, replacing him after the convention is uncharted and potentially messy territory. “The rules are vague, and there is no precedent,” said Benjamin L. Ginsberg, a lawyer with Jones Day who has worked on multiple Republican presidential campaigns.

Naming someone would be hard enough. Finding a suitable alternative was always the problem with the “Never Trump” movement. No matter how distasteful party leaders found him, they could never agree on anyone else.

Gov. Mike Pence of Indiana, Mr. Trump’s running mate, would seem to have a strong claim to make because he is the only other Republican formally nominated by his party. But what if others, rivals of Mr. Trump from the primaries like Senator Ted Cruz of Texas, decided they deserved a shot? A new candidate would almost certainly not be chosen by acclamation. And determining who and how many are eligible would be a huge fight — one the Republican National Committee would have to resolve before it could ultimately vote.

Time Is Running Out

Given that the process of replacing the Republican nominee would probably not go smoothly or quickly — what has for Republicans this year? — there would be very little time left for other essential aspects of a presidential campaign.