Jonathan Ernst / Reuters

The next time a Republican politician claims to put principle before partisanship, ask what he or she did when Representative Justin Amash came under attack.

The Michigan Republican, elected in 2010 at the height of the Tea Party wave, has always presented himself as a libertarian-leaning adherent of limited government. And he has always voted exactly as his supporters would expect, as reflected in the 100 percent lifetime score given to him by FreedomWorks.

But over the weekend, he broke with the GOP on the matter of Special Counsel Robert Mueller’s report: After studying the redacted version and related information, he declared his belief that impeaching President Donald Trump is warranted. How would Republicans react to an honest politician staying loyal to the Constitution as he understands it?

Retired Representative Joe Barton passed this test of character. “There’s no member of the Freedom Caucus more intellectually honest than he is,” he told my colleague Elaina Plott. “So my guess is that he agonized over this and felt like he couldn’t, in good conscience, not say something and still be honest to his oath of office.”

House Minority Leader Kevin McCarthy, however, did not pass the test.

“You’ve got to understand Justin Amash,” he told Fox News. “He’s been in Congress quite some time. I think he’s asked one question in all the committees that he’s been in. He votes more with Nancy Pelosi than he ever votes with me. It’s a question whether he’s even in our Republican conference as a whole.”

Those are flagrant lies.

As Jonathan Chait notes, “A very quick YouTube search turns up a number of instances of Amash asking questions in committees: here, here, here, and here.”

Matt Welch, the editor at large of the libertarian magazine Reason, puts the record straight: “He takes his job with a seriousness that has almost vanished from the legislative branch. He holds the modern day congressional record for most consecutive votes not missed, 4,289 over six-plus years, and reportedly wept when he accidentally missed one.”

As for the content of those votes, Amash has been in line with Trump’s position 91 percent of the time in the current Congress and 61 percent of the time overall, according to FiveThirtyEight. He aligns with the American Conservative Union 82 percent of the time.

But McCarthy is hardly alone in publicly disgracing himself to align with Trump on this matter.

As the libertarian Robby Soave writes at Reason, “Amash’s principled stance has already earned him a primary challenger in the form of an unabashedly pro-Trump state representative named Jim Lower, who called Amash ‘out of touch’ with voters. Turning Point USA President Charlie Kirk attacked Amash as well, smearing him as un-American in a bizarre and conspiracy-minded tweet.

Even the other members of the House Freedom Caucus, the most freedom-loving, freedom-supporting members of Congress, have sided against one of their own to please an erratic big-government authoritarian.

Trump frequently transgresses against the stated principles of most Republican politicians. To stay loyal to him, they must either keep quiet or betray themselves. That so many choose the former is unfortunate, if commonplace. But it is shameful when they react to a colleague who does speak up by savaging him in the press.

Serving Trump is now the GOP’s priority. Principled conservatives should withhold support from the party until he is gone.

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