The Senate on Tuesday agreed to take up a measure that would force President Trump to end the war in Iran or win authorization from Congress to continue it, after a handful of Republicans joined Democrats in pushing forward with a resolution the G.O.P. has managed to block for months.
Senator Bill Cassidy, the Louisiana Republican who lost his primary over the weekend after Mr. Trump targeted him for defeat, was the latest member of his party to switch his vote and side with Democrats in an effort to limit the president’s war powers. That, combined with the absences of several other Republicans, was enough to push the resolution forward.
The vote was 50 to 47 to advance the resolution, allowing it to be debated and receive a vote in the coming weeks. It was the eighth attempt by Democrats and a single Republican to rein in Mr. Trump’s war powers since he began the military campaign, now in its third month, which a majority of Americans say he should never have launched.
Senator John Fetterman of Pennsylvania was again the only Democrat to vote with Republicans to beat back the measure, while Mr. Cassidy was one of four Republicans who sided with Democrats to push it forward. Mr. Cassidy said after the vote that the Trump administration had “left Congress in the dark on Operation Epic Fury,” referring to the administration’s name for the operation.
“In Louisiana, I’ve heard from people, including President Trump’s supporters, who are concerned about this war,” he added in a statement. “Until the administration provides clarity, no congressional authorization or extension can be justified.”
The sliver of G.O.P. skepticism to the president’s handling of the Iran conflict widened last week, fueled in part by Mr. Trump ignoring a statutory deadline to seek permission from Congress to carry on combat operations past 60 days. In both the House and Senate, efforts to advance a war powers resolution were narrowly defeated.