“I can say without any hesitation that the Democrats will retake the Senate,” said Senator Harry Reid of Nevada, the departing Senate minority leader who is not impressed by efforts to retool the Trump campaign. “They’ve been trying to get him to run a real campaign but that’s not who he is.”

Top Republicans admit they face a real challenge.

“We are very much on defense this cycle,” Mr. McConnell told me during the Republican National Convention last month. “These are really, really tough competitive races.”…

A similar dynamic is at work in the House. At the moment, chances for Democrats’ winning the 30 seats they would need to take back power are considered slim in the absence of a complete Trump swoon.

But both sides expect Democratic gains, and the loss of even half that number of Republicans would greatly complicate the life of Mr. Ryan — provided he would even want to remain speaker at that point or that a majority of remaining House Republicans would want him.