“Nope. Donald Trump.”

And so he began to shift his position. At first, he dipped in a toe, declining to criticize Mr. Trump when given the chance. Then, when he saw dissenting Republicans defeated in primaries or humiliated in tweets, he waded in deeper, publicly coming to Mr. Trump’s defense. Now, he’s undergone a baptism. When I recently asked him about Mr. Trump’s behavior, he told me about the strength of the economy, the reduction in taxes and regulation, and the danger of extremist Democrats.

How did he go from “Never Trump” to “never allow a witness who might implicate Trump”?

He’s part of a growing wing of the Republican Party: the rationalizers.

I’m familiar with political rationalization — I watched and even practiced it during my time in Congress. Like erosion, political rationalization is incremental, shaped by harsh winds and climates over a period of time. It usually begins when a small issue meets a larger electoral consequence. Constituents push you to vote “yes” when you want to vote “no,” so you rationalize: “I’ll wait for someone else to put it over the top, then my ‘yes’ vote won’t mean as much.” Or: “I’ll vote yes, but the Senate will defeat it. Or the president won’t sign it. Or if it does become law, well, there’s always the Supreme Court.”

Pressured by re-election, you make a tiny, inconsequential concession on a specific issue or vote. Nothing more than a small chip off your principles.

Early in my career, I had my own moment of rationalization. In 2001, only six months after being sworn into the House of Representatives, President George W. Bush proposed significant tax cuts. I was deeply concerned about their impact on the debt, but my polling showed that voting against lower taxes in my Republican-leaning districts was politically fatal. So I rationalized: “This will be the last tax cut I support” (it wasn’t); “the deficits will be offset” (they weren’t); “by voting with Republicans on this, it gives me more credibility to vote against them on other issues” (actually, in that case, it did).