From Michigan to Louisiana to California on Friday, rank-and-file Republicans expressed mystification, dismissal and contempt regarding the instructions that their party’s most high-profile leaders were urgently handing down to them: Reject and defeat Donald J. Trump. Their angry reactions, in the 24 hours since Mitt Romney and John McCain urged millions of voters to cooperate in a grand strategy to undermine Mr. Trump’s candidacy, have captured the seemingly inexorable force of a movement that still puzzles the Republican elite and now threatens to unravel the party they hold dear. In interviews, even lifelong Republicans who cast a ballot for Mr....