More than 50 prominent figures and former officials have signed a letter urging Senate Majority Leader Mitch McConnell (R-KY) and his colleagues to end America’s involvement in the Saudi-led war in Yemen.

The signatories — including two former US ambassadors to Yemen, leading legal scholars, top Saudi Arabia experts, a Nobel Peace Laureate, a former key aide for Secretary of State Colin Powell, and MIT linguist Noam Chomsky — want McConnell and other senators to support a bill that could forever remove US troops from the Yemen war, which will likely be put to a vote on Wednesday.

“[D]irecting the President to halt all offensive activities alongside Saudi Arabia against the Houthis unless such actions are first approved by Congress ... would spell the likely end to the broader conflict,” reads a copy of the letter obtained by Vox, which is due to be delivered to Senate leadership on Tuesday.

The lead authors of the letter, legal experts Bruce Ackerman and Laurence Tribe and former US ambassadors to Yemen Barbara Bodine and Stephen Seche, argue that Trump’s support for the war is unauthorized by Congress and therefore illegal. Ackerman and Tribe advise the Congressional Progressive Caucus which has pushed the Senate and House hard to end America’s role in the Yemen war.

They support a bill co-sponsored by Sens. Bernie Sanders (I-VT), Mike Lee (R-UT), and Chris Murphy (D-CT) that would force President Donald Trump to stop backing Saudi Arabia, the UAE, and other coalition members in their fight against the Houthi rebels within 30 days. Sen. Chuck Schumer (D-NY), the top Democrat in the chamber, is facing pressure from activist groups like MoveOn to co-sponsor the bill.

If Trump wants to recommit troops to the war, he would then have to seek authorization from Congress to do so. The US helps the Saudi-led coalition by providing them with intelligence, selling them arms and ammunition, and until recently fueling planes in the conflict that has left tens of thousands dead, and millions more suffering from starvation and disease.

The bipartisan bill invokes the War Powers Resolution of 1973 (WPR), which states that if US troops are involved in “hostilities” abroad “without a declaration of war or specific statutory authorization, such forces shall be removed by the President if the Congress so directs by concurrent resolution.”

“Successive administrations have basically defined the word ‘hostilities’ out of existence, and so Sen. Sanders believes it’s important that Congress reasserts itself and engages in a more open and robust debate about where, when, and how our nation fights in war,” a Sanders aide told me.

The WPR also allows a simple majority in Congress to strike down any military action, which means it would pass with a 51-49 vote.

In March, the same bill failed in a 55-44 vote. But if the bipartisan bill passes this week, it would be the first time in US history that the Senate voted to say America’s involvement in a war was unconstitutional.

It’s very likely that a similar House bill, also based on the WPR, would then pass in January when the Democrats take over. At that point, Trump does have the ability to veto the measures — but it could prove politically costly to do so. Follow-up congressional action and lawsuits would almost assuredly take place, as it would appear that Trump aimed to expressly defy a mandate from lawmakers.

What to watch for ahead of, during, and after the vote

There are a few things to watch for related to this week’s vote.

First, Secretary of State Mike Pompeo and Defense Secretary Jim Mattis will brief all senators in a classified setting on Wednesday morning about America’s involvement in the conflict in Yemen. Two congressional sources told me that McConnell specifically asked for the briefing ahead of the vote because he was “concerned” that the Sanders resolution would pass. One of those sources said the White House asked McConnell to request the briefing. A spokesperson for McConnell acknowledged the briefing but wouldn’t comment on anything else.

It’s therefore possible that the briefing will compel Republicans to vote as a bloc against the measure.

However, some high-level Republicans may vote for the Sanders-backed bill. The one to watch is Sen. Joni Ernst (R-IA), who recently became a member of Republican Senate leadership. Over the weekend she rebuked Trump by saying the president has been too soft on Saudi Arabia both for its war in Yemen and for the murder of journalist and dissident Jamal Khashoggi in October. It’d be jarring to see a top Senate Republican openly defy not only Trump but also McConnell.

And finally, it’ll be important to watch how Trump reacts if both the Senate and House pass their bills to end America’s participation in the Yemen war. Trump could veto both bills but he would certainly face an open congressional revolt. At that point, the situation could turn into a crisis as lawmakers seek to assert their ability to authorize America’s wars.

Still, this week could show just how dissatisfied Congress really is with the Yemen war and America’s relationship with Saudi Arabia as a whole — and how Trump has handled both.