It is a bigger lift than it might seem. Mr. Cruz appears keenly aware of his charm deficit, acknowledging in private that his retail campaigning skills can lag behind his grasp of policy. At a debate in October, he became perhaps the first candidate in modern history to declare himself unappealing bar company.

“If you want someone to grab a beer with, I may not be that guy,” he said, when asked to name a weakness. “But if you want someone to drive you home, I will get the job done and I will get you home.”…

Elsewhere, though, Mr. Cruz’s disposition has rarely been described warmly. A college roommate has said he would prefer as president anyone else in the United States, chosen at random from a phone book. In 2013, three months into Mr. Cruz’s tenure, Foreign Policy magazine identified him as both “the most hated man in the Senate” and “the human equivalent of one of those flower-squirters that clowns wear on their lapels.”

Relations have not improved. Congressional colleagues are as likely to cite his arrogance as his ideological tilt when explaining their disdain. Some have likened a private conversation with Mr. Cruz to watching Fox News without any cameras. There is no off switch, no apparent thirst for collegiality.

“Ronald Reagan had a personality that enabled him to persuade people of diverse ideologies to support his point of view,” said Josh Holmes, the former chief to staff to Senator Mitch McConnell of Kentucky. “Ted Cruz persuades people who share his ideology to oppose his point of view.”