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 direction on a conflict that has spanned well over two months.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 either to gain congressional approval or withdraw U.S. 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.The 50-47 vote tally showed that a small but growing number of Republicans are willing to challenge Trump on the Iran war, even though the effort may not advance much further. Three Republicans were absent Tuesday and their votes would be enough to defeat the measure, if they maintain their stance on the war.Still, the vote showed how Republicans are increasingly uneasy with a conflict that shows no signs of ending, is stuck in a fragile ceasefire and is causing rising gas prices in the U.S."Republicans are starting to crack, and momentum is building to check him," Senate Democratic leader Chuck Schumer said in a statement after the vote. "We are not letting up."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 supported the legislation for the first time, while Sen. John Fetterman of Pennsylvania was again the only Democrat to vote against it.After his primary election loss last week, Cassidy returned to Washington defiant. He said 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 going forward.Trump's grip on the Senate Republican Conference is potentially slipping after Cassidy's primary loss, as well as the president's decision Tuesday to endorse Republican Sen. John Cornyn's primary opponent."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 on social media."Until the administration provides clarity, no congressional authorization or extension can be justified."House will also vote on war powersAcross the Capitol, the House is expected to vote on a similar war powers resolution Wednesday, and Democrats are bullish about their chances of passing it. Another war powers resolution barely failed in the House last week, the vote ending in a tie.Even if Congress eventually passes legislation to compel Trump to withdraw from the conflict, it's not clear that he would comply. To circumvent requirements of the War Powers Resolution of 1973, the White House has claimed that it has technically ceased "hostilities" with Iran because there is a ceasefire.Trump this week said he ordered preparations for an attack on Iran on Tuesday, only to call it off to give allies in the Gulf more time to work out an agreement with Iran."Peace negotiations are stuck and so day after day after day grocery prices climb, gas prices climb," Democratic Sen. Chris Murphy of Connecticut said in a floor speech.Lawmakers want more information on Iran warOn Capitol Hill, patience is clearly running out for a war that Trump launched without congressional approval.Republican Sen. Mike Rounds of South Dakota said he is supportive of Trump's decision to go to war, but that more Republicans are growing concerned about a long-term plan. The administration may have to go into more detail about that, Rounds said.The War Powers Resolution of 1973 "does provide an avenue for that discussion and debate to occur," Rounds said.He said he believes it's not the right time to do that, and he would rather "stand strong with the president" at this point. "But I think a number of our members maybe just feel like it's time to have the debate."Earlier this year, Republican senators voted to advance a war powers resolution on the conflict with Venezuela and compelled the Trump administration to make its case to Congress for the military campaign.A handful of GOP senators forced a final vote on legislation to withdraw from the conflict. Two of the Republicans — Sens. Todd Young of Indiana and Josh Hawley of Missouri — ultimately flipped after Secretary of State Marco Rubio agreed to a public hearing about the administration's Venezuela strategy.__Associated Press writer Mary Clare Jalonick contributed to this report.