The House on Wednesday was set to vote on a measure directing President Trump to withdraw U.S. forces from Iran or win approval from Congress to continue the war.
The resolution had been on track to pass in May when Republican leaders abruptly pulled it from the floor to avoid an embarrassing defeat for both the party and the president.
Adoption of the resolution would be a remarkable rebuke to Mr. Trump and his handling of the war, after he has repeatedly dismissed any effort by Congress to curb his power and the G.O.P. has largely ceded its prerogatives to do so, deferring to him time and again. Republicans put off the vote two weeks ago, recognizing that they did not have sufficient votes to defeat the measure.
But it was not clear whether they had made any headway in the days since, in winning converts, as the conflict has dragged on and Mr. Trump has made little progress toward ending it. And G.O.P. leaders are unable to delay the vote any longer because Democrats have invoked the War Powers Resolution, which requires consideration of such measures within a limited period of time.
Three House Republicans sided with Democrats in support of a similar resolution that just barely failed on a tie vote last month, in a sign of growing opposition to the military campaign now in its fourth month.
The vote comes as divisions between Republicans in Congress and the president have surfaced on a range of issues as their interests diverge in the run-up to the midterm congressional elections. Senate Republicans have in recent days forced Mr. Trump to abandon his request for $1 billion in security funding for his ballroom project and a plan that the Justice Department announced to create a federal fund to pay claimants who accuse the government of having victimized them.
Even if the resolution were to pass and be approved by the Senate, the ability of lawmakers to force a president to withdraw troops remains a contested legal question, and Mr. Trump and his senior aides have dismissed any effort by Congress to limit his war powers as unconstitutional.
But a Senate vote last month to move forward with a similar measure, in which a handful of G.O.P. defectors broke from the president and opposed the war, indicated an increasing willingness by some members of the president’s party to pressure him to end a conflict that a majority of Americans say is not worth the costs.
