WASHINGTON — The Senate voted 50-47 on Tuesday to move forward with a resolution to force President Donald Trump to end the war in Iran, a breakthrough for the Democratic-led effort.

Sen. Bill Cassidy, R-La., who just lost his primary for renomination over the weekend after facing opposition from Trump, voted “yes” to advance the measure, the first time he has done so after repeatedly voting “no” in previous instances.

“While I support the administration’s efforts to dismantle Iran’s nuclear program, the White House and Pentagon have left Congress in the dark on Operation Epic Fury,” Cassidy said in a statement. “In Louisiana, I’ve heard from people, including President Trump’s supporters, who are concerned about this war. Until the administration provides clarity, no congressional authorization or extension can be justified.”

The other Republicans who crossed party lines to support the procedural motion were Sens. Rand Paul, R-Ky., Lisa Murkowski, R-Alaska, and Susan Collins, R-Maine, all of whom have voted aye before.

“Vote by vote, Democrats are breaking through Republicans’ wall of silence on Trump’s illegal war,” Senate Minority Leader Chuck Schumer, D-N.Y., said after the vote. “For more than 80 days, Trump has dragged America into a costly, chaotic conflict with no plan, no objective, and no legal authority. Today proved our pressure is working: Republicans are starting to crack, and momentum is building to check him. We are not letting up.”

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Every Democrat voted in favor of the war powers resolution, apart from Sen. John Fetterman, D-Pa., who has opposed it every time.

Introduced by Sen. Tim Kaine, D-Va., the resolution states that “Congress hereby directs the President to remove the United States Armed Forces from hostilities within or against Iran, unless explicitly authorized by a declaration of war or a specific authorization for use of military force.”

It’s unclear when the Senate will hold a final vote on a war powers resolution. And it’s not yet guaranteed to pass: Three senators were absent for Tuesday’s vote — Sens. John Cornyn, R-Texas, Thom Tillis, R-N.C., and Tommy Tuberville, R-Ala. — and if all three vote against it, it would break 50-50, and fail.

Several Republicans have voiced unease with the ongoing war, and the latest vote means that one more GOP senator in the “yes” column would mean it passes.

It would still need to pass the Republican-controlled House, however, and would almost certainly face a presidential veto.