President Donald Trump was unhappy about signing a $1.3 trillion omnibus spending bill last month, because Democrats wouldn’t agree to his demands for an astonishingly high level of military spending without also reversing the historically low public investment in the domestic budget. So he had a temper tantrum on national television. He unleashed a stream-of-consciousness rant (with Trump, is there any other kind?) about the “ridiculous situation” he was in. He called on Congress to give him a line-item veto, which the Supreme Court has already ruled unconstitutional. And while he fumed about the horrible omnibus that nobody read, he mentioned in passing that he signed it, reversing an empty veto threat. And we all had a good laugh.

It was an oddly calming moment, restoring temporary faith in the idea that coequal branches of government exist and sometimes a president can do nothing but bluster. It even demonstrated that Democrats can use their limited power in Washington and secure what looked like a victory for the public. But that wasn’t the end of the story.

Trump and House Republicans, furious about having to work with Democrats, have concocted a scheme to renege on the deal they made, according to Roll Call. It would involve using an obscure post-Watergate law to pass legislation cutting an unspecified amount of domestic spending out of the omnibus bill. And they can do it without needing a single Democratic vote.

This cruel, direct cut to vital domestic programs that have been underfunded since the deficit craze kicked up in 2010 could be very damaging to Republicans politically, particularly in swing districts. But that’s only if they go the legal route to accomplish it, putting Congress on the spot to deliver a vote. And Donald Trump’s not someone likely to let the law get in the way of an idea.

The Beltway name for this procedure is “impoundment,” and it was first used by President Thomas Jefferson, who in 1801 refused to spend $50,000 appropriated by Congress to buy Navy gunboats. Presidents throughout history have left appropriated funds unspent, using various rationales. But Richard Nixon, who we can safely assume is Donald Trump’s role model in this gambit, took it a step further, routinely impounding funds for projects he simply didn’t like.

The Supreme Court ruled some of Nixon’s actions illegal, and Congress responded in 1974 with the Impoundment Control Act. If a president wants to rescind spending on a certain program, they have to notify Congress specifically, with an estimate of the fiscal impact and a reason for the impoundment, and then get legislative approval. Congress would have to assent to the rescission request within 45 legislative days. Importantly, rescissions cannot be filibustered, which means that Congress could approve such a request with a simple majority vote in both chambers.

The prospect of passing budget cuts without Democratic support—the omnibus needed 60 Senate votes—has led House majority leader Kevin McCarthy and the Trump administration to collaborate on working through the contours of a rescission scheme, according to Roll Call.

No details have been released on what might actually be cut. But just comparing the omnibus to Trump’s budget requests shows that the programs on the chopping block would be ones that assist the most vulnerable people in society. Trump’s budget proposal would have slashed Head Start for needy elementary-school students and Pell grants for needy college students. It wouldn’t have increased a low-income housing tax credit that Democrats earned in exchange for fixing a glitch in the tax law. It would have suppressed the budgets of the Environmental Protection Agency and the National Institutes of Health and would have transformed food stamps into a box of food delivered to homes.

This would result in a bait-and-switch: dealing with Democrats to secure insane amounts of funding for the military, and then turning around and reversing the domestic spending that Democrats got in the deal. It’s something no president has really done since the Impoundment Control Act was enacted, and for good reason.

First of all, the Senate is closely divided at 51-49, and with John McCain not expected back anytime soon, Republicans would have no margin for error if they wanted to pass this. It would also be one of the more unpopular bills you can imagine—a direct assault on needy families and important protections, just months before the midterm elections. It verges on political suicide.

There’s also the point that Democrats would have to be crazy to ever work on a single bipartisan bill ever again during the Trump administration if it pulls this trick. No deal would be safe from a behind-the-back reversal. What’s more, it would offer a road map to Democrats to engage in countless rescissions of spending they don’t like, if and when they returned to control Congress and the White House. This would be the legislative equivalent of hemlock.

But implicit in this analysis is the assumption that Trump would go through the proper procedures to impound omnibus spending. As budget guru Stan Collender explains, “The key question won’t be whether Trump will obey the law, it will be whether he just decides not to spend what he doesn’t want to spend.”

Trump has already created a precedent for this. The State Department was supposed to spend $120 million to counter foreign meddling in US elections. It didn’t. The administration had authority under a near-unanimous congressional law to sanction Russia for election-related conduct. It didn’t, Defying congressionally mandated appropriations and laws would be sadly routine for this president. And if Republican leaders in Congress didn’t say anything about Trump’s not spending what they appropriated, there would be no recourse to stop this Nixonian gambit.

I think the idea of Trump being a dictatorial president gets thrown around in a cavalier manner sometimes, but defying congressional spending authority by nullifying programs of national importance would certainly take us down that road. It would centralize practically all governing power in the hands of one rather unstable man. Democrats need to make this toxic scheme widely known, and fast.