Washington — The Senate advanced a resolution to limit President Trump's war powers in Iran on Tuesday, marking a breakthrough for Democrats after seven failed attempts.

Senators approved a motion to discharge the resolution from committee in a 50 to 47. Four Republicans — Sens. Susan Collins of Maine, Lisa Murkowski of Alaska, Rand Paul of Kentucky and Bill Cassidy of Louisiana — joined the bulk of Democrats in favor of advancing the resolution. Sen. John Fetterman of Pennsylvania was the sole Democrat to oppose. 

The vote marked the first time Cassidy supported advancing a war powers resolution. It came days after he failed to win enough support to advance to a runoff in the Louisiana GOP Senate primary, where Mr. Trump endorsed one of his opponents. 

And three Republicans — Sens. John Cornyn of Texas, Tommy Tuberville of Alabama and Thom Tillis of North Carolina — did not vote, tipping the scales in Democrats' favor for the first time since they began bringing war powers resolutions related to Iran.

The resolution, led by Democratic Sen. Tim Kaine of Virginia, would direct the president to "remove the United States Armed Forces from hostilities within or against Iran, unless explicitly authorized by a declaration of war or a specific authorization for use of military force." 

The vote marked only a first step in the Senate. And even if both chambers approved the resolution, the president would be expected to veto it. But Democrats say the move would carry significance, and have the potential to change the president's thinking in the war. 

Kaine told reporters ahead of the vote that he couldn't predict whether it would attract additional GOP support, noting that he "didn't know that we would get an additional vote last week" when Murkowski became the third Republican to support advancing the resolution. 

"But I do know this: I know what we are all hearing from our constituents, and that is, they are deeply opposed to this war," Kaine said.

The vote came as the president announced Monday that the U.S. wouldn't follow through with "scheduled" attacks on Iran on Tuesday, though he noted that he was "an hour away" from making a decision to order the new strikes. Kaine said the threat makes it "the perfect time to do what we should have done in February, and have a congressional discussion about rationale, plan, straits."

The Virginia Democrat also pointed to the war's economic toll and the high gas prices that are impacting Americans. 

"We're coming up on Memorial Day. A lot of people do a lot of driving, they're going to be paying a lot for gas — a whole lot more than they paid last year, and they remember that," he said. 

Kaine said the public's lack of support for the war "is really hardening right now."

"And that will be what eventually gets us to pass the resolution here," Kaine said. "Is it there yet? I don't know, but it is coming."

Senate Minority Leader Chuck Schumer urged Republicans to "be honest with themselves" ahead of the vote Tuesday. 

"Last week we got one more Republican to join with us," Schumer said. "Republicans, it's time to break the cycle — support our war powers resolution. No president, no president should be able to drag this country into war alone."