“We’re at an interesting inflection point in American politics,” he said in an interview. “If somehow dissent from your own party becomes viewed as a bad thing, then we’re not really vetting and challenging ideas in the way the founding fathers intended.”

Broadening his argument, Mr. Sanford said America was meant to be “a nation of laws, not men” and that “we weren’t a cult of personality.”

Mr. Sanford said he recognizes that Republicans in his district, which Mr. Trump carried by 14 points, want him to line up with the president, and cited a survey saying that he had voted with Mr. Trump in Congress 89 percent of the time. “I love my brother and sister, but I don’t agree with them 89 percent of the time,” he said.

Still, his opponent, Ms. Arrington, has made extensive use of clips of Mr. Sanford taking aim at Mr. Trump. She argued in a debate this month that “our first job is to listen to the captain” and that “Mark Sanford has spent the better part of two years bashing our captain.”

Ms. Roby, by contrast, has not staked out a position of anything approaching regular dissent; she essentially opposed Mr. Trump in public only for a short period near the end of the 2016 presidential campaign, over his personal behavior. After a recording surfaced of Mr. Trump making vulgar comments about women, she said he was “unacceptable as a candidate for president” and urged him to step aside.

“When she came out against Trump, the people down here in this part of the state — oh my God, they hate it,” said Will Matthews, a Republican lawyer in Ozark. “She showed her true colors to kowtow to the traditional Republican Party people.”