“I don’t agree with his strategy,” said Deb Fischer, the senior Nebraska senator, whose manner is generally as mild as her state’s farmstead cheese. “I’m not going to hand this election to Hillary Clinton.”

Mr. Sasse is unbowed. “In private, many people will say some version of ‘I wish I could take the position you’ve taken, but I don’t know how that will affect me back home,’ ” Mr. Sasse said during an interview in his Senate office. “I don’t know where it’s written that fighting for what you believe in will always be popular with 51 percent.”…

Mr. Sasse, an evangelical Christian, easily quotes Scripture without notes. Last month, at a Family Research Council event in Washington, Mr. Sasse contextualized his political theology.

“There is a great risk in our time that our people who are scared and who are disoriented will vest too much in politics, and they will believe that their American-ness is the center of their identity,” he said. “And I am here to tell you, brothers and sisters, that is idolatrous.”