Donald Trump Jr. on Friday morning gleefully shared a new poll. It showed the first Republican to call for President Trump’s impeachment, Rep. Justin Amash (R-Mich.), suddenly trailing a primary challenger. “See you soon Justin,” Trump Jr. wrote on Twitter, signaling he’ll campaign for Amash’s opponent. “I hear Michigan is beautiful during primary season.”

Amash’s response: Bring it on.

“If it’s what you say I love it especially later in the summer,” Amash said, echoing Trump Jr.'s infamous response to being offered opposition research from a Russian lawyer in 2016.

The response encapsulates the basically unprecedented nature of Amash’s continued attacks on President Trump. While other Republican lawmakers have occasionally criticized Trump in strong terms, they’ve almost always quickly been brought to heel. Many of them saw their poll numbers among Republicans tumble, as Amash now has, and apparently decided it wasn’t worth the trouble.

But Amash is proving to be a unicorn in his own party: a Republican willing to stand up to Trump and actually stand by it in no uncertain terms. And he’s apparently willing to tank his own political career in the process.

Examples of Republicans who stood up to Trump and then backed down are myriad. There is Sen. Thom Tillis (R-N.C.), who claimed a principled stand against Trump’s border wall national emergency — and then flip-flopped in the face of intraparty pressure. Sen. Dean Heller (R-Nev.) once declared himself to be “99 percent against Trump” but then embraced him ahead of a tough 2018 reelection bid. Sen. Bob Corker (R-Tenn.) mended fences with Trump after saying the president was “debasing” the country. And even Trump’s most vocal Senate critics — Sen. Mitt Romney (R-Utah), former senator Jeff Flake (R-Ariz.) and the late senator John McCain (R-Ariz.) — often measured their criticism, including avoiding attacking Trump directly.

The common thread running through almost all of them? Declining poll numbers and huge pressure from the base. Heller lost, Corker and Flake retired, and Tillis is on the ropes. Further proving the rule is the inverse: After calling Trump crazy and unfit for the presidency in 2016, Sen. Lindsey O. Graham (R-S.C.) has bear-hugged him of late — and seen his approval rating among Republicans rise from 51 percent in April 2018 to 74 percent less than a year later.

The political calculus for would-be Trump critics is exceedingly simple: Every apostasy will be punished, so you better pick your spots when it comes to principled stands. Amash has thrown caution to the wind, though, doubling down on his positions even as his party ratchets up the pressure. The new poll shows him trailing state Rep. Jim Lower (R) 49 percent to 33 percent.

Amash quit the tea-party-aligned House Freedom Caucus, which he co-founded, and House Minority Leader Kevin McCarthy (R-Calif.) this week suggested Amash wasn’t philosophically aligned with his party anymore. He also deflected when asked if Amash should leave the GOP conference.

Through it all, Amash has continued making the same case. He has tweeted extensively about why he thinks Trump obstructed justice and has criticized Attorney General William P. Barr for “deliberately misleading” about the Mueller report.

If anybody was a candidate to stake out this position and stick to it no matter the personal cost, Amash was it. He’s long been known as an exceedingly principled libertarian Republican who has departed from the GOP on certain issues. But that kind of lawmaker is exceedingly rare in a business that rewards survivalists and risk mitigators. Indeed, politics selects for people who are willing to convince themselves that expedience is okay — if not wholly justified — over the long term. Whatever you think of Amash’s cause, it’s remarkable that he’s apparently fully willing to lose his job over this.

There is, of course, another option, and that’s running as the Libertarian Party’s nominee for president. It’s a move that could throw a real wrench in the 2020 race, given Amash hails from one of the most important states on the map and would potentially take a larger share of the vote there. Even doing that, though, would almost undoubtedly be the end of his time in elected office. At best, he would be running a statement candidacy that could spoil the election for one candidate or another. Imagine running for president knowing there was a good chance you could be the next Ralph Nader.

If one lawmaker were willing to do that, though — even if just for the principle of it — Amash might be the one.