The vote on a procedural measure to advance the resolution was passed on Tuesday by a margin of 50 to 47, with a handful of Republicans joining Democratic colleagues to pass the measure in a rare rebuke of the US president, Al Jazeera reported.
The vote showed that a small but growing number of Republicans are increasingly uneasy with a war that shows no signs of ending, amid a fragile ceasefire, and are willing to challenge the president.
“This president is like a toddler playing with a loaded gun,” Democratic Senate Minority Leader Chuck Schumer said before the vote.
“If there was ever a time to support our war powers resolution to withdraw troops from hostilities with Iran, it’s now,” he said.
The result was also a victory for lawmakers who have been arguing that Congress, not the president, should have the power to send troops to war, as spelled out in the US Constitution.
However, it was only a procedural vote, and the resolution faces steep hurdles if it is ever to go into effect. Three Republicans were absent during the vote on Tuesday, and their votes would be enough to defeat the measure if they maintain their stance in favor of the war.
But even if the resolution is eventually passed by a vote of the 100-member Senate, it must also pass the Republican-led House of Representatives and garner two-thirds majorities in both the House and Senate to survive an expected Trump veto.
Trump’s Republicans have blocked seven previous attempts to advance similar resolutions in the Senate this year. They have also stopped three war powers resolutions by narrow votes in the House this year.
Yet, the vote on Tuesday underscores growing pressure on the president, as the war on Iran, launched by the US and Israel in late February, continues to wreak havoc on the global energy market and the cost of living at home.
Democrats and a few Republicans have called on Trump to come before Congress and seek authorization for his war, with some expressing concerns that the president may have entered the US into a long conflict without setting out a clear exit strategy.
The Trump administration claims the president’s actions are legal and within his rights as commander in chief, as well as his responsibility to protect the US by ordering limited military operations.
Under a 1973 US war powers law passed in response to the Vietnam War, a US president can wage military action for only 60 days before they must end it, ask Congress for authorization, or seek a 30-day extension, due to the “unavoidable military necessity regarding the safety of United States Armed Forces” while withdrawing forces.
Trump declared on May 1 that a ceasefire with Tehran had “terminated” hostilities, meaning he had not waged more than 60 days of war on Iran.
Opinion polls suggest that US voters are opposed to the war, which legal experts widely consider to be a violation of international law.