These loyalties have upended the Tea Party-versus-establishment divide, which has dominated fratricidal primary seasons since 2010 but increasingly has the air of fins on the back of a car, a quaint relic from an earlier era. With Mr. Trump in charge, the political market for purity on the right has been devalued.

“Because of the loyalty that the most conservative voters in our base have to Trump, there’s a pull there that’s scrambling the ideological lines,” said Andy Sere, a Republican ad man who has been heavily engaged in the party’s internecine wars. “He wants wins on the board, and that’s traditionally been the goal of the pragmatists.”

The shifting conservative fault lines are on display in the affluent and mall-dotted northern suburbs of Atlanta, which were at the front end of the South’s political realignment in the 1970s when they turned away from their Democratic roots and elected a loquacious young college professor named Newt Gingrich to Congress.

The special election on April 18 has drawn substantial attention because one of the Democrats running, Jon Ossoff, has raised a remarkable $8 million, and his success in a Republican-leaning district could presage a midterm backlash against Mr. Trump. (Voters will pick from candidates of both parties on a single ballot; if no one clears 50 percent, the top two finishers will advance to a runoff election.)

But the way the Republican hopefuls are running is just as instructive. Even as they try to win over the sort of conservative activists dedicated enough to participate in a rare April election, the Republicans are casting themselves more as can-do pragmatists in the spirit of Mr. Trump than unwavering ideologues. At the forum and in individual interviews afterward, three of the Republicans in the 18-person field invoked some variation of Ronald Reagan’s maxim that it is better to get 80 percent of what you want than nothing at all.