The Senate voted on Tuesday to advance a war powers resolution aimed at forcing Donald Trump to end the war in Iran unless he receives congressional authorization to continue it.

Tuesday’s 50-47 vote marks the first time the chamber has advanced the bill, the eighth attempt at doing so since the conflict began in February.

Four Republicans joined all but one of the Senate’s Democrats in voting to pass the bill.

This time, Senator Bill Cassidy, fresh from a primary loss in Louisiana in a race where Trump endorsed his opponent, voted to take up the measure.

“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 wrote on social media after the vote. “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.”

Lisa Murkowski of Alaska, Rand Paul of Kentucky and Susan Collins of Maine also voted to advance the bill, as they did in previous votes. John Fetterman of Pennsylvania was the sole Democrat to vote against it, while three Republicans did not vote.

“Vote by vote, Democrats are breaking through Republicans’ wall of silence on Trump’s illegal war,” the Senate minority leader, Chuck Schumer, said in a statement. “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.”

Tuesday’s vote marks just the first step toward full passage of the resolution, which Trump will almost certainly veto even if it does pass in both the Senate and the House.

Still, Democrats see the resolution’s advancement as a way to force Republicans to take a stance on Trump’s unpopular war.

“The Senate should use this moment to do what we should have done before the war started – discuss the rationale, strategy, end state, and costs to American taxpayers and our economy,” said the bill’s sponsor, Virginia senator Tim Kaine, in a statement before the vote.