Mr. Sessions’s predicament says a lot about a Republican Party that Mr. Trump has turned into a vessel for his own political security, held together not by shared beliefs but instead by fealty to him. No one was more of an evangelist for the ideas Mr. Trump ran on than Mr. Sessions. Yet his political fate is now threatened because the president has declared him disloyal.

“We live in a time now when if you don’t fall on your sword for the president, you’re done,” said John Castorani, a candidate for Congress from the Mobile area and a self-described moderate Republican, who said Mr. Sessions did the right thing by recusing himself. “I was proud to be from Alabama — for a moment,” added Mr. Castorani, a 27-year-old veteran and former intelligence officer.

“When we criticize Trump, we are no longer patriots, we are country-hating liberal hacks,” he said. “I’m not going to fall on my sword for him. And if that keeps me from getting elected, I’m O.K.,” he said.

The negative effect of Mr. Trump’s barrage against Mr. Sessions became clear in interviews with 20 Alabama voters. Most brought up the recusal with no prompting. Many said they held it against their former senator, though some admired him for sticking to his principles. And even those who couldn’t recall what exactly Mr. Sessions did had heard enough to understand that whatever happened was bad for the president.

“I know he did something that made the president mad,” said Susan Woodman, a retired speech therapist who came away from the Huntsville event undecided but impressed with one of Mr. Sessions’s rivals, Tommy Tuberville, a former Auburn University football coach.

Mr. Tuberville introduced himself to the audience by saying his biggest reason for wanting to serve in Washington was “to help Donald Trump” — be it on trade or foreign policy or fighting political correctness in schools that he complained teach “climate change, diversity and all that crap.”

All of the candidates have pledged in no uncertain terms to help Mr. Trump and his agenda if elected. Mr. Sessions’s bet is that he can claim something more convincing: He was on board with that agenda first.