But the votes of other senators have become more elusive. They have come to believe that their constituents, even the most conservative ones, are more loyal to them than to Mr. Trump…

“Right now, nobody’s afraid of Trump, and that’s a real problem,” said Rob Jesmer, the former executive director of the National Republican Senatorial Committee and once a top aide to Senator John Cornyn of Texas.

“But the truth is that he hasn’t really tried,” Mr. Jesmer added. “Where is he on local talk radio? Where is the trip to Kansas to say, ‘Hey, Jerry, we’re really close on this and could use your help’? It’s what he does well, getting out there and making the case. I don’t get why he hasn’t been more engaged.”