President Trump has declared a national emergency in order to pay for physical barriers along the U.S.-Mexico border, but will Congress block it? That’s a long shot — but we can’t rule it out either.

Here’s the basic process, as the New York Times explained in an article after Trump’s declaration. Congress can take up a resolution to end a presidential national emergency declaration. If such a resolution passes in one chamber, the other must bring it up for a vote within 18 days. If the resolution passes both chambers and the president vetoes it, a two-thirds majority in Congress can override that veto.

In the House, Democrats say they are strongly considering a resolution to override Trump’s emergency declaration. If they take it up for a vote, the resolution is almost certain to pass — Democrats have a 235-197 majority in the House.

That would move the legislation to the Senate, where Republican Leader Mitch McConnell — although initially wary of Trump declaring a national emergency — has now expressed his support for the move. But there does not appear to be a way for him to avoid a Senate vote on this measure. And such a vote will force Senate Republicans, many of whom have said they are wary of presidents overextending their power at the expense of Congress, to choose between that principle of limiting executive power and backing a president with strong support among party activists on one of his signature initiatives, the border wall.

Republicans have a 53-47 advantage in the Senate, so only four defections and this resolution would pass. (The resolution is not subject to rules that would require 60 votes for it to move forward.) By FiveThirtyEight’s count, at least eight GOP senators — Tennessee’s Lamar Alexander, Maine’s Susan Collins, Alaska’s Lisa Murkowski, Kentucky’s Rand Paul, Florida’s Marco Rubio, Nebraska’s Ben Sasse, North Carolina’s Thom Tillis and Pennsylvania’s Pat Toomey — have said they oppose the national emergency declaration. Opposing the declaration and voting for a resolution to end it are not the same thing, but more than a dozen other Republican senators have also expressed reservations about Trump’s move.

Yes, we know that in the past GOP senators have hinted that they will break with Trump on key legislation and then not followed through. That said, we are talking about only four senators needing to break with the president on this resolution. And three of the Republicans to come out against the declaration — Collins, Murkowski and Paul — were key figures in stopping the GOP from repealing Obamacare in 2017, so they have in at least one major instance actually followed through on their concerns with a Trump initiative.

In other words, there’s a real chance that both houses of Congress pass this legislation. A bill adopted by the GOP-controlled Senate rejecting Trump’s emergency declaration would be important symbolically. And it would force Trump to issue the first veto of his presidency, which will bring even more attention to his already unpopular move to declare a national emergency.

What happens if Trump vetoes the resolution?

In the Senate, 67 votes would be needed to override his veto, meaning 20 Republicans would need to break with the president. As noted above, there are more than 20 Senate Republicans wary of this move. So it’s at least theoretically possible that the dam could break; the fissures run deeper than the typical issues on which only a handful of Republicans like Collins and Murkowski are considering a vote against the president. But not liking the emergency declaration and voting for a resolution that will stop it — and stoking Trump’s anger as many Republicans are gearing up for primaries in 2020 — are totally different things. It’s also not clear if McConnell is required to even hold a vote on the veto override.

The bill would also have to achieve a two-thirds majority in the House, which wouldn’t be easy. FiveThirtyEight has not done a formal whip count in the lower chamber. A two-thirds majority would require 288 votes — so 53 Republicans would have to join all the Democrats. Most Republicans in competitive districts lost in 2018, which means there are relatively few GOP representatives left who have to worry about electoral pressure. Earlier this year, amid the government shutdown, for instance, a bloc of about a dozen Republicans broke with Trump to back some Democratic measures to fund the government without wall money. So it’s not like all 197 House Republicans are Trump loyalists. But a dozen is a long way from the 53 GOP votes in the House to override his veto.

It’s still more likely the courts, not Congress, are the place where this national emergency declaration faces its most serious challenge. But the congressional process is worth watching too. Much of Trump’s agenda (such as confirming conservative judges to federal courts) is broadly supported by GOP legislators. In fact, you could argue that much of what has happened in Washington over the last couple years represents Trump embracing the congressional Republicans’ agenda, not the other way around. But the border wall is not really a major priority for McConnell or many key congressional Republicans. Trump cares deeply about it. So the president, by issuing this declaration, has forced his party into what amounts to a loyalty test — will they stand with him, even if it means abandoning some of their long-held concerns about executive overreach?

UPDATE (Feb. 19, 2019, 9:56 a.m.): The table in this article has been updated to add Sen. Cindy Hyde-Smith’s position. Hyde-Smith’s office responded to a FiveThirtyEight request for comment.