The Senate advanced legislation Tuesday that seeks to force President Donald Trump to withdraw from the Iran war, as a growing number of Republicans defied the president’s wishes.

Since Trump ordered the attack on Iran at the end of February, Democrats have forced repeated votes on war powers resolutions that would require him to either gain congressional approval for the war or withdraw troops. Republicans had been able to muster the votes to reject those proposals, but Louisiana Sen. Bill Cassidy — fresh off a primary election loss in which Trump endorsed his opponent — switched sides to deliver a crucial vote to advance the legislation.

The 50-47 vote tally demonstrated the small but crucial number of Republicans voting to halt the war with Iran. The legislation will get a vote on final passage, but the timing was not immediately clear. There were also Republican Senate absences Tuesday that would be enough to defeat it, if those lawmakers maintained their stance on the war.

Still, the vote showed how Republicans are increasingly uneasy with a conflict that is in a fragile ceasefire and has caused rising gas prices in the U.S.

Republican Sens. Rand Paul of Kentucky, Susan Collins of Maine and Lisa Murkowski of Alaska had all previously voted for similar war powers resolutions and did so again Tuesday. Cassidy voted for the legislation for the first time.

After his primary election loss last week, Cassidy returned to Washington saying that he was proud of his work to uphold the Constitution and would carefully consider how he would vote on several priorities of the Trump administration.