The unassuming governor of Indiana, Mr. Pence is in a powerful, if unenviable, position. Were he to publicly repudiate his own running mate, or question his fitness for office, the Trump campaign would be unsustainable. He does not need to take such a drastic action — not yet — but the prospect of his doing so, even if conveyed obliquely, might persuade his running mate to broker a withdrawal. At the very least, it might spur intermediaries, such as Mr. Trump’s friends and family, to have a candid conversation with the candidate on what lies ahead if he stays in.

The nominee has spent a lifetime equating the Trump name with success. Now he faces the prospect of a staggering defeat — recent polls suggest he could lose even in reliably Republican states like Arizona and Georgia. If such an outcome were to occur, he would be forever branded, fairly or not, racist, sexist and, worst of all in the Trump lexicon, a historic loser.

But were Mr. Trump to leave the race now, on whatever pretext, he could state honestly that he bested a formidable Republican establishment and that he brought important issues like illegal immigration to the fore. He would win the gratitude of his party for putting its interests first. He could help his capable children rebuild the tarnished Trump brand. And with a little luck, his running mate, should he replace Mr. Trump as the nominee, might defeat Hillary Clinton, who has severe image problems of her own. In that event, Mr. Trump could reasonably boast that he hand-selected the next president.

If Mr. Pence truly believes Mr. Trump will be a capable president, then he should do nothing. But he owes it to his party, his country and the cause he has championed his entire life to reflect on this carefully. The worst-case scenario is that a man who may be wholly unfit for the office may actually win it. Mr. Pence is among the few who could stop this — now. And, if he has any question about whether Donald Trump can do the job, then he has a responsibility to step aside himself if Mr. Trump won’t.

Though it is against his nature and probably his instincts to turn against his running mate, Mr. Pence must put the country first. Besides, if the party chose him as Mr. Trump’s replacement, and if he managed to receive Mr. Trump’s blessing, Mr. Pence could yet salvage this mess. He could command support from the sizable Trump faction, establishment figures like Senator Mitch McConnell and Representative Paul Ryan, a personal friend, and fellow conservatives like Senator Ted Cruz, whom Mr. Pence originally endorsed. He could select a vice-presidential nominee who would broaden the ticket.