Mr. McConnell for years pushed Mr. Trump’s agenda and only rarely opposed him in public. But the message that he delivers privately now is unsparing, if debatable: Mr. Trump is losing political altitude and need not be feared in a primary, he has told Mr. Ducey in repeated phone calls, as the Senate leader’s lieutenants share polling data they argue proves it.

In conversations with senators and would-be senators, Mr. McConnell is blunt about the damage he believes Mr. Trump has done to the G.O.P., according to those who have spoken to him. Privately, he has declared he won’t let unelectable “goofballs” win Republican primaries…

Even more pointedly, Mr. McConnell vowed that if Senator John Thune of South Dakota, the second-ranking Senate Republican, faces the primary that Mr. Trump once promised, Mr. Thune “will crush whoever runs against him.” (The most threatening candidate, Gov. Kristi Noem, has declined.)