“If you criticize him, you’re siding with the media that hates him and you’re undermining what he’s trying to accomplish,” said former Senator Rick Santorum, explaining why Mr. Trump has consolidated support. “Trump has done a good job in being a conservative president and as much as Democrats looked past what Bill Clinton said and did, Republicans are looking past what this president says and does.”

That means Republican lawmakers are going to be further bound to Mr. Trump in the midterm elections, less likely to raise doubts about his impulsive policymaking and, perhaps most consequential for a president facing scrutiny by a special counsel, more inclined to refrain from criticizing his personal conduct and use of executive power.

“This will have a further chilling effect,” said Mr. Flake, adding, “If you want to do well in a primary, you’ve got to act accordingly.”