Striking at the party wasn’t the best move for Mr. Trump. If he wants to be elected and to govern successfully, he should not have attacked congressional Republicans. He has little standing to preach party loyalty, having only recently joined the GOP. He funded Democrats for decades and made a special effort to elect Nancy Pelosi speaker in 2006.

If Mr. Trump wants a Republican Senate to approve his nominees and a Republican House to pass his agenda, then he should give GOP candidates the freedom to do what they must to win. Almost every one of them is polling ahead of him. He needs their coattails to get to the White House.

Mr. Trump’s comments undoubtedly thrilled the “alt-right” leaders of his campaign, who want to burn down the Republican Party perhaps even more than they want Mr. Trump to win. But swing voters are less than impressed. As of Wednesday, Mrs. Clinton’s lead had grown to 6.2 points, 48% to 41.8%, in the Real Clear Politics average of polls.