When Justin Amash became the first sitting Republican congressman to call for President Donald Trump’s impeachment— the Michigan lawmaker believes special counsel Robert Mueller’s report reveals obstruction of justice — it triggered new speculation that he will run for president as a Libertarian.

It’s not clear Amash will fulfill these wishes — he’s been equivocal when asked, but has never closed to the door. If he does decide to take the presidential plunge in 2020, however, he shouldn’t do it as a Libertarian; he should challenge Trump as a Republican.

Amash’s own political career is testament to the fact that everything libertarians have accomplished electorally, save for the stray local race, has come through the Republican Party. He is a five-term congressman today because of his affiliation with the GOP. As a third-party candidate, he would have been a spoiler at best.

Gary Johnson had respectable showings as the Libertarian presidential nominee in 2012 and 2016, eclipsing the party’s previous high-water marks in the popular vote. As a Republican, however, he was the two-term governor of New Mexico. Ron Paul finished a distant third when he was the Libertarian candidate for president in 1988. As a Republican, he was elected to Congress 12 times.

Even Paul’s failed runs for the Republican presidential nod were more consequential than his successful bid for the Libertarian nomination. He proved a candidate who opposed the Iraq war could gain at least some traction with GOP voters. He spawned a legion of like-minded Republican candidates, including a few prominent election winners, such as his son Rand, now serving in the Senate, Kentucky Rep. Thomas Massie — and Justin Amash.

Could Amash have a similar impact if he ran in the Republican primaries? Barring some unforeseen event that causes the bottom to drop out for the president, like an economic downturn or scandal that actually moves public opinion, any challenge to Trump will almost certainly fail. It may even be a career-ending failure for anyone who tries it. The end result could be a lot closer to John Ashbrook or Pete McCloskey’s asterisk candidacies against Richard Nixon in 1972 than Ronald Reagan’s effort to unseat Gerald Ford four years later.

But Amash would undoubtedly be more effective, and get more media coverage, in this role than Bill Weld, a two-term Republican governor of Massachusetts turned Libertarian also-ran now well past his prime. Unlike Weld or Maryland Gov. Larry Hogan, the anti-abortion Amash could appeal to some of the more socially conservative Never Trumpers, a neglected but important part of that shrinking coalition (a pro-life candidate could do a lot better against Trump in Utah than a pro-choice one, for instance).

Amash could credibly run against the return of trillion-dollar deficits under Trump and the failure, once again, of unified Republican control of the federal government to rein in the swamp. Instead we’ve seen the Grand Old Spending Party return, just like when George W. Bush and Denny Hastert soaking taxpayers in red ink.

The congressman could also press Trump on his failure to end the wars he campaigned against in 2016. The withdrawals from Afghanistan and Syria, which Amash supports, are stalled. Amash voted for the resolution to end U.S. backing of the Saudi war in Yemen, which Trump vetoed. Tensions are increasing with Iran and Venezuela as other presidential wars continue unabated.

Amash, who has repeatedly opposed Trump’s border wall funding requests, wouldn’t be as well positioned to run against Trump’s failure to keep his campaign promises on immigration, possibly the president’s biggest liability with Republican voters after the issue was his greatest strength four years ago. But Amash at least stops short of Gary Johnson’s “no caps, no quotas” position.

It would also be easier for Amash to continue as a Republican in Congress, if his district wants him to, if he doesn’t bolt the party in the presidential race. And while he has been denounced by everyone from House Minority Leader Kevin McCarthy to his own Freedom Caucus, another “libertarian moment” could be right around the corner. The election of a Democratic president always seems to bring the small-government wing of the Republican Party back in vogue, and that’s when that faction’s strength in Congress will actually matter.

If Trump does end up losing, it would be better for Amash to have faced him as a Republican. The argument that Pat Buchanan drew first blood against George H.W. Bush in 1992 with his strong showing in New Hampshire did not prevent him from competing in the primaries four years later. Ralph Nader was permanently discredited in the minds of many progressives for playing third-party spoiler in 2000.

Of course, Trump could win reelection. And either way, Amash’s candidacy would likely run into the brick wall of the president’s popularity with a GOP electorate that seems less interested in smaller government than a government that will protect them from big tech, a hostile media, social liberalism and economic dislocations in culturally conservative areas. But if the appeal of limited government is, well, limited, it’s going to be a long four years for Justin Amash anyway.