Lee and Paul eventually backed the resolution, after a briefing from top administration officials about the killing of General Qassem Soleimani left them deeply dissatisfied. And on Tuesday, Kaine got additional support from Republican Sens. Todd Young of Indiana and Susan Collins of Maine, giving him the four Republican votes he needs to pass the measure.

Senate passage of a proposal to rein in the president's war powers represents a remarkable rebuke, even if there likely aren't enough votes to withstand Trump’s veto.

Kaine’s effort highlights the Virginia senator’s long obsession with keeping checks on presidential war powers. The one-time vice presidential hopeful notes that he also called for President Barack Obama to come to Congress when it came to the war with ISIS or possibly striking Syria for the use of chemical weapons. That position, Kaine says, has helped him earn trust in his conversations with Republicans when it comes to this president.

“Republicans know I pressured Obama on this,” Kaine said, adding he “wouldn’t get very far” if he had only pressured Trump. “They know I’m on the Armed Services Committee, they know I pressed Obama as much as I press Trump, they know I got a son in the military”

Senate Republicans, when asked about Kaine, also like to highlight his criticism of Obama.

“While President Obama was still in office he was willing to step forward and express concerns where others weren’t,” Lee said. “I like that.”

Young, who described Kaine’s initial draft of the resolution as “politically charged,” said the Virginia lawmaker’s consistency gives him credibility on the matter — “a consistency few others bring to the issue.”

“The other thing that gives him credibility is he has approached this without — to my knowledge — taking political potshots at the president, and that stands in stark contrast to some others,” the Indiana Republican added.

Kaine said there are about 10 to 12 Republicans who could possibly back the War Powers Resolution. But it’s likely to be met with resistance from most members of the Senate GOP, who have raised concerns about the resolution’s timing amid recent tensions with Iran and fear further restricting Trump’s authority.

Sen. Mitt Romney (R-Utah) said in a statement Tuesday that he will oppose Kaine’s resolution, arguing it would “send the wrong message to Iran” and “tie the President’s hands in responding to further potential Iranian aggression.”

Sen. Kevin Cramer (R-N.D.), a strong Trump ally, said the resolution was “political gamesmanship” and that “anything that Donald Trump does, the response from Democrats - you should be suspicious.”

And Collins, who initially had concerns about the message Kaine’s resolution would send to Iran, had qualms about some of the earlier language, particularly when it came to removing troops from hostilities. Kaine, however, helped convince her by clarifying language that the resolution aimed to “terminate the use of U.S. forces in hostilities." That change appeared to appease Collins.

The Maine Republican said Monday, prior to announcing her support for the resolution, that Kaine was “very receptive,” adding he “made changes that in my view improve the resolution.”

Kaine initially drafted the War Powers resolution three months ago. The latest version of his resolution can come to the Senate floor as early as next week. But the timing for the resolution remains in limbo given the impeachment trial. It’s unclear yet whether the Senate will vote on legislation while the trial takes place. Once it passes the Senate, it will go to the House.

The resolution is not the first time the Senate has asserted congressional authority over Trump’s foreign policy. Last year, the Senate passed a bipartisan War Powers resolution to end U.S. support for the Saudi-backed war in Yemen, which Trump vetoed.

The chamber also voted on an amendment to the defense authorization bill from Kaine and Sen. Tom Udall (D-N.M.) that would require Trump to seek congressional approval for military action in Iran. The amendment, however, didn’t meet the 60-vote threshold to pass.

Kaine recalled that at the start of his tenure in the Senate, few senators were interested in discussing Congress’ role in war powers. He says that’s changing now because of two factors: the belief that Trump is impulsive and the amount of time that’s passed since Congress authorized the military use of force in 2001 and 2002 in the aftermath of the 9/11 attacks.

“I felt very lonely — like nobody was interested in this when I started, and more and more people are now interested in this issue,” he said. “The longer we're in this state of perpetual war ... the people see that there's some impulsive action they're like, 'we gotta, we gotta do something to clean this up.'”

