In interviews, even lifelong Republicans who cast a ballot for Mr. Romney four years ago rebelled against his message and plan. “I personally am disgusted by it — I think it’s disgraceful,” said Lola Butler, 71, a retiree from Mandeville, La., who voted for Mr. Romney in 2012. “You’re telling me who to vote for and who not to vote for? Please.”

“There’s nothing short of Trump shooting my daughter in the street and my grandchildren — there is nothing and nobody that’s going to dissuade me from voting for Trump,” Ms. Butler said.

A fellow Louisiana Republican, Mindy Nettles, 33, accused the party of “using Romney as a puppet” to protect itself from Mr. Trump because its leaders could not control him. “He has a mind of his own,” she said. “He can think.”

The furious campaign now underway to stop Mr. Trump and the equally forceful rebellion against it captured the essence of the party’s breakdown over the past several weeks: Its most prominent guardians, misunderstanding their own voters, antagonize them as they try to reason with them, driving them even more energetically to Mr. Trump’s side.