This post is updated with Trump’s reaction.

Rep. Justin Amash’s decision to form an exploratory committee for a run for president as the Libertarian Party candidate could wind up being a very significant moment in the 2020 race.

Exactly why and how, though, isn’t entirely clear.

Amash has been toying with a third-party run since a well-documented rift between him and the Republican Party, which he left symbolically on July 4 to become an independent. Amash, who says he will now change his party affiliation to Libertarian, was a founding member of the tea-party-oriented House Freedom Caucus, but his politics have always skewed more libertarian, and he said he became frustrated with the GOP over time. He went on to support the impeachment of President Trump, even as all of his former party mates in the House voted against it.

The case for him being a significant presence in the 2020 race is readily apparent. He’s not just a sitting congressman and former Republican, but he also hails from one of the most important states in the 2020 election: Michigan. The state went for Trump by just 0.2 percentage points in 2016 — the thinnest margin of any state. That means even a modestly strong showing by a Libertarian nominee could theoretically swing the state and potentially the race for presidency.

Exactly to whose benefit could that accrue, though?

Being a former Republican with conservative fiscal policies would seem to potentially attract GOP-leaning voters who are disillusioned with Trump but perhaps unwilling to move over to the presumptive Democratic nominee, former vice president Joe Biden. Amash also represents one of 14 Michigan congressional districts — and a conservative-leaning one at that, with the district going for Trump in 2016 by 10 points. A relatively strong showing (by third-party standards) in his home area could help him gain significant votes statewide. Libertarian nominee Gary Johnson, himself a former Republican governor, took 3.6 percent in the state in 2016.

But there are also plenty of reasons to believe that Amash, assuming he’s not actually competing for a win, could wind up helping Trump.

A poll conducted in May by the Detroit News, for instance, showed that Biden led Trump in Michigan by 12 points in a head-to-head race, 53 to 41. But adding Amash to the equation reduced Biden’s margin by half, with Biden leading 45 to 39 and Amash pulling a significant 10 percent.

Amash also drew 16 percent of independent voters in that poll, reducing Biden’s 13-point lead among them to a tie with Trump.

That poll is from a long time ago, and we regrettably don’t have any more recent, high-quality polls testing an Amash bid either in Michigan or nationally, so it’s difficult to say that’s how things stand today.

But there is other evidence that the presence of a formidable third-party candidate more generally could accrue to Trump’s benefit.

One of my favorite poll numbers is how the “double-haters” vote. When Trump was running against Hillary Clinton in 2016, each candidate was liked by only about 4 in 10 voters. That made for a large universe of voters who didn’t like either — and those voters favored Trump significantly and likely decisively.

As Philip Bump wrote earlier this month, exit polls show they went for Trump by 21 points in Michigan, 25 points in Pennsylvania and 37 points in Wisconsin, way more than accounting for his slim margin in each state.

Although those double-haters went for Trump in 2016, there is significant evidence that they favor Biden this time around. A Quinnipiac poll this month showed they favored Biden by 32 points nationally — similar to his margin in December in the same poll.

The margin was about the same in a Fox News poll in August, too, with these voters favoring Biden 43 percent to 10 percent.

These are, importantly, small sample sizes. That’s because the universe of voters who dislike both major-party candidates in 2020 is, thus far, smaller than it was in 2016, when we had two of the most unpopular nominees in modern history. Still, Biden’s advantage among them is well-documented and remarkably consistent across these polls.

And what they suggest is that voters who are disillusioned with both of their top options in 2020 are primed to help Biden significantly and have very little interest in voting for Trump. Giving them a formidable third option — especially in a state such as Michigan — could logically mean that that costs Biden more than Trump.

Trump seemed to welcome Amash into the race in a tweet Wednesday morning, casting him as unable to win reelection to the House as an independent. He also suggested he might hurt the Democrats more than Green Party candidate Jill Stein did in 2016. (It’s, of course, difficult to take such musings about whether other candidates should run at face value.)

All of that said, it’s super early. While casual voters might have heard of Amash, they probably don’t know much about him besides his support for impeachment and his departure from the GOP. If he runs a serious campaign and people come to understand his platform and his tea party roots, perhaps that would be more attractive to conservative voters who might otherwise support Trump.

While the conventional wisdom is that Trump’s base is small but completely devoted to him, polling has suggested there is some softness to it. Many supporters say Trump isn’t honest and trustworthy, or say that he doesn’t make them “proud” that he’s president.

So even as many anti-Trump conservatives denounced Amash’s exploratory bid Tuesday night as well-intentioned but counterproductive, it’s difficult to say that with any certainty.

It’s all a big what-if at this point, but what’s abundantly clear is that it’s something worth keeping an eye on.