There is significant exposure in airing even the most mild criticism of the president, as Mr. Bozell was reminded the other day when he pointed out on Twitter that China, which Mr. Trump had just congratulated on its 70th anniversary as a communist republic, was a repressive regime.

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“The fury is absolutely there for anyone who criticizes this president,” he said. Still, he offered nothing but scorn for the few remaining Never Trump Republicans, whom he accused of being self-righteous and politically shortsighted. “For a lot of the purists, they would rather go down in flames than look at any political equation,” he said. “These are the people who supported George W. Bush when he did nothing for conservatives, and they don’t have any leg to stand on when it comes to passing judgment on Trump.”

Mr. Bozell has also discovered that there is a significant market for defending Mr. Trump against impeachment . Through the organization he founded, the Media Research Center, he has helped provide the Trump-friendly news media with a steady stream of videos and articles alleging bias in the mainstream news media’s coverage of impeachment. He has also co-written a book this year on a similar theme: “Unmasked: Big Media’s War Against Trump.”

In the Republican national security community, many still openly criticize Mr. Trump. But some of the most prominent signatories of the 2016 letter have taken a more charitable view of the president today. John Negroponte, the former director of national intelligence, said earlier this year: “I certainly don’t think his presidency has been catastrophic.” Tom Ridge, the former Homeland Security secretary, said that his concerns about Mr. Trump have not diminished over time, but has found occasional areas of agreement on policy, such as the need for a border wall.

And at least one of those officials who signed the letter, James Jeffrey, now works for the Trump administration. He has a high-profile posting in the State Department as the special representative dealing with Syria.

The motivations for getting on board are considerable: a job, a bigger audience, a white knight-like belief that you can change things from the inside. “Some of this is pure opportunism and careerism,” said Eliot A. Cohen, a State Department official in the second Bush administration who signed the letter and also helped recruit like-minded Republicans to join an earlier anti-Trump letter that called the then-candidate “fundamentally dishonest.”