Senate Republicans are threatening to give President Trump their biggest legislative rebuke to date: a vote to cancel his proposed escalating tariffs on goods imported from Mexico. The Washington Post reports that Senate Republicans believe they have a veto-proof majority to do that.

While a threat is often just a threat (my colleague Amber Phillips has detailed the various reasons this may never come to light), it’s worth emphasizing how much damage it could do to Trump’s efforts, even if it’s never carried out.

Republicans have thus far been willing to humor Trump about his trade wars. Few of them have ever been fans of tariffs, which party orthodoxy generally considers to be “taxes.” But they have largely balked at doing much of anything as Trump has ratcheted things up, given his sway with the base.

Trump has continually upped the ante, especially on China. And given that there seems to be no end in sight for his thirst for tariffs — he apparently believes his strategy is winning the trade war with China — this was a conflict likely to come to a head sooner or later.

Senate Republicans have apparently realized that Trump wants to go further than they can stomach, and they’ve decided they have no choice but to do something to send a message. Trump has set a deadline of June 10 for Mexico to meet his conditions, which include halting the flow of migrants headed to the U.S.-Mexico border, before the tariffs will increase by 5 percentage points each month. Needless to say, that’s a situation that could get very serious very quickly, especially since Mexico sends 80 percent of its exports to the United States.

What’s so remarkable about the Senate GOP’s threat is not so much that they are actually standing up to Trump, which is rare, but also that they are severely diminishing his leverage in the process. If you are Trump, you need Mexico to believe that these tariffs will be enacted next week, so they come to the table and give you what you want. If senators from Trump’s own party are saying they will block the tariffs regardless, Mexico has far less incentive to concede much of anything. Why not just wait and let those senators do their thing?

And even if Trump and Mexico are able to reach a deal to avert the tariffs — and the need for Congress to block them — it seems quite possible that the deal will be quite a bit less favorable than it might otherwise have been. If you are Mexico, you’re not going to give away the farm. And if you’re Trump, there is more incentive to come to some kind of a deal to save face and avoid your own party throwing a wrench in the gears of your trade war. That could impact future potential tariffs as well, not just the ones on Mexico.

The fact that this is spilling out into the open is also telling. These threats could just as easily be made privately, as the deadline nears and Trump attempts to win concessions from Mexico. Why not let the deadline approach, put up a united front and give Trump his momentary leverage? The fact that Republicans aren’t even willing or able to do that suggests they don’t see much of an alternative.

We’ll see how this process plays out over the next week. But unlike many Republican rebukes of Trump that have been rather toothless, this one actually carries some bite — even if they never have to go through with it.