Some third-party activists are happy to run the risk of a Hillary victory as long they can guarantee that Mr. Trump loses. But such a motivation won’t sit well with the millions of Republicans who have voted for Mr. Trump. Those voters presumably believe the New Yorker can win in November, but if he doesn’t they need to see the consequences of their primary vote.

The GOP would have a hard enough time recovering from a third-straight presidential loss. The last thing the party needs is an excuse for Mr. Trump and his allies to blame a defeat on a “stab in the back” by other Republicans. That’s a recipe for more civil war and another fiasco in 2020. If Mr. Trump does lose, his voters need to understand that he was the architect of his own demise. Republican voters also need to see that alienating non-whites, women and young people was a losing strategy.

Then there are the down-ballot risks. Advocates say a third-party candidate would motivate GOP voters who might otherwise stay home in November if Mr. Trump is the nominee, and these voters would then support GOP House and Senate candidates.

Perhaps, but it’s more likely that dueling presidential candidates would put House and Senate Republican candidates in a perilous spot. Do they support Mr. Trump or the third-party conservative?