The US Senate on Wednesday blocked a bill that would have stopped President Donald Trump continuing the military campaign against Iran without congressional approval.
The vote was 52 to 47, mostly along party lines.
This is the latest attempt by Democrats to block Mr Trump from engaging American forces in conflicts abroad. A similar bill was put to a vote after the capture of Venezuelan leader Nicolas Maduro in January. That also failed.
Before the vote, Republican senator Lindsey Graham likened the conflict with Iran to stopping the rise of Nazi Germany.
"How many chances did you have to stop Hitler, and you just blew it because you thought he really didn't mean to kill all the Jews, that he could be reasoned with?" he asked. "We're not going to do that again."
Mr Graham accused Iran of being run by "religious Nazis" and added that the US would bring "them down with our Arab partners".
"The Arabs in the region have been terrorised by the Iranian regime, and they're going to join the fight, I hope soon," he said.
Mr Graham also railed against what he called an "unconstitutional" rule that requires a president get congressional approval to start an armed conflict that lasts longer than 60 days.
Under the US Constitution, only Congress has the power to declare war, but he said that the President is the commander-in-chief and therefore is the one who should possess the power to engage the country in a conflict. He said that Congress could instead express its disapproval for a conflict by cutting off funding.
Democratic Senator Chris Murphy opposed Mr Graham's claims that compared the current conflict to a battle between good and evil.
"Not everything is World War Two," he said, adding that trillions of US taxpayer money has been spent on regime change action abroad. "When are we going to learn? … Six American troops have already died in an illegal war that nobody wants."
Mr Murphy added that the region is in chaos, while "American consumers are paying the price".
"And for what? We still don't know the reason for this war."
A sister resolution in the House of Representatives was expected to be put to a vote on Wednesday.
