Spurred by Donald J. Trump’s mounting victories, a small but influential — and growing — group of conservative leaders are calling for a third-party option to spare voters a wrenching general election choice between a Republican they consider completely unacceptable and Hillary Clinton…

“I would sooner vote for Josef Stalin than I would vote for Donald Trump,” said Mr. Boot, who expressed optimism that Mr. Trump could still be defeated. He added: “There is no way in hell I would ever vote for him. I would far more readily support Hillary Clinton, or Bloomberg if he ran.”

Among religious conservatives, too, anxiety about Mr. Trump is spreading. Russell Moore, head of the political arm of the Southern Baptist Convention, said he had been deluged by evangelicals asking his guidance on what to do in a race between Mr. Trump and Mrs. Clinton.

A Trump-Clinton race would have no palatable choice, Mr. Moore said. He said it would be impossible for him to support any candidate “who stirs up racial animosity” or supports abortion rights.