The Senate on Tuesday adopted a resolution instructing President Trump to end the war in Iran or seek congressional authorization to continue it, delivering the most significant bipartisan rebuke yet of the conflict.
The resolution does not have the force of law and is therefore unlikely to compel an immediate change in policy. But the 50-to-48 vote — in which four Republicans joined Democrats in favor — marked a striking break by the G.O.P.-led Congress with a president who has faced little resistance from his party on any topic, particularly matters of war and national security.
It came as Republicans in Congress have expressed skepticism and alarm about the cease-fire agreement Mr. Trump struck with the Iranians. And it underscored a growing impatience among them as the war was approaching its fifth month about continuing to defer to the president, who has never sought approval from Congress for the war, while further negotiations over its end appear precarious and Mr. Trump has threatened more military action.
The vote was also the latest evidence of tension over the war inside the Republican Party, which faces a punishing political environment ahead of midterm elections in which G.O.P. control of Congress is at stake. With polls showing the conflict deeply unpopular, some lawmakers in the party have voiced concerns about its economic toll, uncertain objectives and the risk of a broader regional escalation.
Tuesday’s vote marked the first time since the enactment of the War Powers Resolution of 1973 that both chambers of Congress have approved a concurrent resolution directing a president to end a military conflict. The House passed the measure this month after Republican leaders who had tried to block it were unable to keep the party unified in opposition.
In the Senate on Tuesday, Senator John Fetterman of Pennsylvania was the lone Democrat to vote against the resolution. Senators Rand Paul of Kentucky, Lisa Murkowski of Alaska, Susan Collins of Maine and Bill Cassidy of Louisiana broke with fellow Republicans and supported the measure. Their backing and the absence of two Republicans who have opposed such measures in the past, including Senator Mitch McConnell, Republican of Kentucky, who was recently hospitalized, allowed the resolution to prevail. The law was born out of a clash between Congress and President Richard Nixon over the Vietnam War, with lawmakers overriding his veto in an effort to reclaim authority over decisions of war.
“The most solemn power for Congress is Congress has the power to declare war, not the president,” Senator Tim Kaine, Democrat of Virginia and a leader of his party’s efforts to win passage of a war powers measure, said ahead of the vote.
What happens next is unclear.
The measure was a concurrent resolution, a vehicle that does not need a presidential signature to take effect but also does not become law. In 1983, the Supreme Court ruled that to have legal effect outside of Congress, legislative actions generally must pass both chambers and be presented to the president for signature or veto.
But supporters of the resolution say war powers measures are different because the Constitution gives the power to declare war to Congress alone. The issue has never been definitively tested before the Supreme Court.
“Whatever happens with this, it’s going to have no effect,” said Senator Jim Risch, Republican of Idaho and chairman of the Foreign Relations Committee, noting the legal questions surrounding the mechanism. Mr. Trump, he added, was not “going to pay any attention to it.”
But the Republican resistance was notable, particularly as the Trump administration is expected as early as Wednesday to request tens of billions of additional dollars to pay for the war.
And Democrats celebrated the vote as an achievement of their goal of registering congressional opposition to Mr. Trump’s unilateral decision to start the war in Iran without authorization. They had tried and failed repeatedly in the Senate to do so before Tuesday’s successful vote.
“This war has been bad for the American people,” Senator Tina Smith, Democrat of Minnesota, said ahead of the vote, casting doubt on the negotiations to wind it down. “Congress had the opportunity to end this war months ago, and we should have done just that to avoid this exact scenario.”
Republicans who opposed the measure argued that it was largely moot, noting that active hostilities had subsided and that the conflict was currently governed by a cease-fire while negotiators worked toward a broader, more permanent agreement.
But despite those talks, there were still 50,000 U.S. troops assigned to the Middle East, officials said, including Army paratroopers, sailors and airmen aboard two aircraft carriers and other naval ships, and more than 5,000 Marines.
And the progress of the discussions between the United States and Iran remained uncertain as conflicting accounts emerged about Tehran’s nuclear ambitions.
Mr. Trump said in a social media post that Iran had “fully and completely agreed to the highest level Nuclear inspections,” a major concession and news that was welcomed by military hawks on Capitol Hill. But that was disputed by Iranian officials who said that the nuclear program had not been discussed during detailed talks held over the weekend in Switzerland.
In the face of the contradictions, Mr. Trump said he would keep troops deployed in the region to facilitate a quick reinstatement of a military blockade of the Strait of Hormuz, “should it be necessary,” though he said that was “highly unlikely.”
The oscillations highlighted the unease among lawmakers over the president’s handling of the conflict, which began with joint U.S.-Israeli strikes on Feb. 28.
Mr. Trump signed a memorandum of understanding between the U.S. and Iran last week at the Palace of Versailles, and Vice President JD Vance has been leading a U.S. delegation in Switzerland for talks on the contentious remaining issues. But Congress has remained sidelined, with top leaders only learning about the memorandum after it was reached and so far receiving scant details on how it would be carried out.
Helene Cooper and Michael Gold contributed reporting.
