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.
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.
On Tuesday, Senators Lisa Murkowski of Alaska and Susan Collins of Maine, both of whom rejected the administration’s claim that the fragile cease-fire between the United States and Iran has pushed off the 60-day deadline, joined Senator Rand Paul of Kentucky, who opposes foreign military intervention, in voting with Democrats to bring the measure to the Senate floor.
It was not immediately clear when the Senate could vote on passage of the war powers resolution, which, even if approved by both chambers, would still be subject to an all-but-certain veto.
With three G.O.P. senators absent — Tommy Tuberville of Alabama, Thom Tillis of North Carolina and John Cornyn of Texas — the majority was unable to beat back the resolution as they have seven times since the war began. Still, Mr. Cassidy’s defection was the latest sign of growing Republican resistance to Mr. Trump’s handling of the conflict and to his refusal to engage with Congress on it.
“The momentum is moving our way slowly,” said Senator Tim Kaine of Virginia, the Democrat leading the weekslong effort to pressure Republicans into voting to end the war.
Democrats have for months argued that passage of such a measure would send a message to Mr. Trump that popular opinion for the operation had soured.
“What the president cares about is his own popularity, and when Congress, even including members of his own party, start to vote against him,” Mr. Kaine added.
The House was expected to vote on a similar measure in the coming days. Lawmakers in that chamber just barely defeated a war powers resolution last week on a tie vote, after two Republicans, frustrated by the president ignoring the legal deadline to seek permission from Congress to carry on fighting past 60 days, defected to join Democrats to move ahead with the measure.
The vote fell as the cease-fire looked increasingly shaky. Mr. Trump said Monday that he would hold off launching any new major attacks on Iran to allow more time for diplomacy. But he has threatened to order a “full, large-scale assault” if Iran does not agree to terms acceptable to the White House.
Disagreement between the United States and Iran over the future of Iran’s nuclear program and the Strait of Hormuz have slowed talks. Iran has mostly barred transit through the major global shipping route since the opening days of the war, driving up the cost of oil and gas and fueling frustration in the United States over the war because of the spike in energy prices.
Mr. Kaine said he expected the Senate would not take the next procedural vote on his war powers resolution until after the Memorial Day recess. He added that he hoped hearing from constituents would make Republicans who voted against it on Tuesday think twice about whether they would continue to stand by the war.
“People are going to hear an earful when they get home about gas prices,” he said.
