The Senate on Thursday narrowly rejected an effort to direct President Trump to end the war in Iran unless he wins authorization for it, defeating a measure aimed at rebuking his handling of a conflict he has continued to wage despite repeated votes by Congress to halt it. The resolution, which passed the House over the summer, failed 50 to 49, with four Republicans — Senators Susan Collins of Maine, Rand Paul of Kentucky, Lisa Murkowski of Alaska and Thom Tillis of North Carolina — joining Democrats in support. Senator John Fetterman of Pennsylvania was the lone Democrat to oppose the measure.

The vote came as a growing number of Republicans has broken from President Trump on the conflict, reflecting voters’ concerns ahead of midterm elections. But Senator Bill Cassidy, the Louisiana Republican who had previously voted with G.O.P. defectors to curtail Mr. Trump’s war powers, switched his position and sided with the majority of his party on Thursday, leaving the resolution short of the support to pass. It was the 20th vote on such a resolution since the war began on Feb. 28.

Another remains. The House passed a separate war powers resolution last week directing Mr. Trump to remove American forces from hostilities against Iran, with seven Republicans joining Democrats in their largest bipartisan margin since the war began. Such a measure would not have the force of law, but would amount to a political rebuke.