The Senate on Tuesday advanced a resolution to halt military action in Iran after a surprise defection from Republican Sen. Bill Cassidy of Louisiana, dealing a blow to President Donald Trump’s war effort.

The vote, though preliminary, shows that the Senate now has the votes to force Trump to pull back the military from Iran or seek congressional approval for additional action.

Despite the War Powers Resolution advancing 50-47, it still has little chance of becoming law. It would need to pass a final vote in the Senate, clear the House and Trump would be almost certain to veto it. But the vote does show increasing headwinds to the war with Iran, especially as gas prices continue to soar ahead of the summer driving season and the 2026 midterms.

Cassidy, who failed to advance to a runoff against Trump-endorsed challenger Rep. Julia Letlow, R-La., in a primary election last week, is now in the final months of his term in the Senate. His vote indicates that he is now more willing to challenge Trump.