Such is the nature of the Republican primary season in the Donald Trump era: self-proclaimed outsiders marketing themselves as loyalists to a man who is now the ultimate insider. Challenges to incumbents from the right are nothing new. But whereas conservative primary candidates once fashioned their campaigns as referendums on reckless federal spending or the elitist sensibilities of leadership, their pitches are much simpler now, carte blanche offerings of complete and total fealty to the president.

And with Mr. Trump’s stranglehold on his party, their targets are much fewer, too. Mr. Meehan’s candidacy is significant not so much on its own merits but in that there are hardly any others currently like it: In this new moment of G.O.P. unity, it is difficult to find a district where one can claim — at least credibly — that the incumbent Republican has been insufficiently supportive of the president.

But complete and total fealty is no longer a sure ticket to Mr. Trump’s support. Hovering over all of these races is the Senate impeachment trial, and how Republican incumbents respond stands to be a critical consideration in how they are viewed by voters — and by Mr. Trump. The president, who once seemed to believe that no slight was too small, no grievance too stale, now appears to regard lawmakers who covet his endorsement through the prism of impeachment alone. And with every House Republican having defended the president, G.O.P. primary challengers are now struggling to successfully argue that their opponents are disloyal.

Pennsylvania’s First is a rare district where support for Mr. Trump is not a given. Mr. Fitzpatrick has voted with Mr. Trump less than any other House Republican, and just 36.5 percent of the time in the current congressional session. In July 2019, he was one of four Republicans to formally condemn the president’s comments about four progressive congresswomen as racist. Which is to say that the Make America Great Again faithful, in their quest to excise yet more of the president’s dissenters from Congress in 2020, smelled blood.