Declarations like these may be important to make and gratifying to read, but they really shouldn’t be coming from those whose integrity depends on them remaining outside the political fray, even in these insane times. For starters, they make it easier for Mr. Trump and his defenders to argue, as they already do, that crucial witnesses in the investigation by the special counsel, Robert Mueller, into the Trump campaign’s possible ties to Russia are biased against the president. Mr. McCabe acknowledged this dynamic in an interview with The Times, describing his dismissal as “part of an effort to discredit me as a witness.” He’s right. But now that he has taken the bait and defended himself publicly — however understandable that is — he has fed that dynamic.

It’s not that Mr. Trump is more credible than these men; to the contrary, it’s hard to think of an American public figure right now with less credibility than the president, who boasted last week about lying to the Canadian prime minister. It’s that this is exactly how Mr. Trump likes it: He drags people down to his level, forcing them to choose between retaliation and silence.

Their decision wouldn’t be so hard if the G.O.P. leadership in Congress were not, as usual, morally absent. Cynical as ever, Mitch McConnell, the Senate majority leader, has said nothing in the face of the president’s degrading tweets and renewed attacks on the Mueller investigation. The House speaker, Paul Ryan, sent a spokeswoman out to issue a milquetoast defense of the special counsel. By keeping mum as Mr. Trump marauds across American democracy, they are forcing the targets of his attacks to defend themselves and also abetting Mr. Trump’s assault on the credibility of the F.B.I. and the Justice Department.

A handful of Republicans have spoken up — the usual suspects, like Senator Jeff Flake, who called the firing of Mr. Mueller “a massive red line that can’t be crossed,” and Senator Lindsey Graham, who repeated his warning to Mr. Trump that it would be “the beginning of the end of his presidency.” On Sunday, Representative Trey Gowdy, who knows a thing or two about long-running investigations, scolded Mr. Trump’s lawyer John Dowd for urging the shutdown of Mr. Mueller’s investigation. “If you have an innocent client, Mr. Dowd, act like it,” Mr. Gowdy said.