Sen. Tim Kaine, a Virginia Democrat, speaks with reporters in the Senate Subway at the US Capitol in January.
Washington CNN  — 

Democratic Sen. Tim Kaine of Virginia is under no illusion about the future of his resolution to rein in President Donald Trump’s authority to launch US military action with Iran.

Trump opposes the language, and while the Senate passed the resolution 55-45 Thursday, the margin is well short of the number needed to override a presidential veto.

But as a key player in a growing coalition of bipartisan lawmakers seeking to retake authority they say has long since been abdicated to the executive branch, for Kaine it’s another step in the process.

It’s a process built on trust and relationships across party lines – an increasing rarity in an almost viscerally partisan chamber – that he hopes, one day, will lead to a rebalancing of how the two branches operate on matters of military action.

“That’s what I’m trying to do,” Kaine told CNN of an effort that brought eight Republicans on board to vote in favor of his war powers resolution. “Hopefully, it will create conditions that will be better for the way the Senate operates.”

The way the Senate operates is, according to senators in both parties, at a particularly tenuous moment in the wake of a bitterly divisive impeachment trial and years of party-line battles and votes. Plunging into a debate over how to shift the congressional role in military action – something that has drifted further into the executive branch’s bailiwick through multiple administration of both parties – seems hardly the way to bring the parties together.

Yet over the course of years and two administrations, Kaine has sought to do just that, both on war powers issues and on authorizations for use of military force, the latter of which, specifically those passed in 2001 and 2002, have all but given presidents carte blanche to launch military action without congressional approval in advance.

Kaine’s latest effort serves as a window into how he’s laying the bipartisan groundwork to, at some point in the future, change the current dynamic. It was sparked by Trump’s decision to strike, and kill, Iranian Gen. Qasem Soleimani, the head of the Islamic Revolutionary Guard Corps’ Quds Force, an elite unit that handles Iran’s overseas operations.

He’s not alone, either.

“We view this as a long-term project,” Republican Sen. Mike Lee of Utah, who was a co-sponsor of the Iran war powers resolution, told CNN. “This is a long-term problem. It’s going to require a long-term effort to restore the separation of powers.”

But the Iran resolution, as with previous efforts related to US operations tied to the civil war in Yemen, or efforts to repeal or rewrite past authorizations for military force, wasn’t a given.

Kaine speaks following the Senate voted on the War Powers resolution, at the US Capitol in Washington, DC on Thursday.

Opposition from the White House

Trump himself tweeted that the resolution “sends a very bad signal,” adding that Democrats “are only doing this as an attempt to embarrass the Republican Party.”

Administration officials worked the phones to try to blunt the effort, multiple congressional aides told CNN.

Senate Majority Leader Mitch McConnell, who strongly opposed the resolution, was stark in his assessment.

“This resolution is not ready for prime time,” the Kentucky Republican said. “I believe it is just an effort to broadcast a political message. But even that message can be harmful to our troops and to national security.”

It’s the kind of statement, with the fraught political undertones attached to it, that lawmakers acknowledge has played a role in their unwillingness to dive deeply into the issue over the course of nearly two decades.

The politics, paired with the sharp ideological divide over executive power and authority as it pertains to national defense and military action – an example of which played out in a fiery exchange between Republican senators on the Senate floor shortly before the final vote Thursday – has had the effect of short-circuiting debates for years.

But it was Kaine’s work – and willingness to make changes to his original resolution – that led to a successful vote this week.

“He’s worked it. He’s made modifications to win votes. He’s trusted as a negotiator,” Sen. Richard Durbin of Illinois, the second-ranked Senate Democrat, who co-sponsored the resolution, told CNN. Asked why Kaine was trusted, particularly at the current fraught moment in the Senate, Durbin replied: “His word is good.”

The effort involved navigating sharply different political and ideological pressure points. There were GOP senators like Lee and Sen. Rand Paul of Kentucky, who are fierce defenders of Trump but ideologically set on reclaiming congressional authority on constitutional grounds, as well as more moderate lawmakers like Sens. Susan Collins of Maine and Lisa Murkowski of Alaska, who operate in a more bipartisan space and agree with Kaine’s topline goals as it relates to congressional authority.

Sen. Todd Young, a conservative Indiana Republican who has carved out space as a key bipartisan foreign policy voice, was a top priority.

The specific views of each aren’t necessarily aligned with Kaine’s directly, or those of many of his Democratic colleagues, something on display at news conferences this week.

Senate Minority Leader Chuck Schumer confidently declared after the Senate had approved the resolution that its passage will serve as a warning to the President.

“The Senate just sent a clear shot across the bow – a bipartisan majority of senators don’t want the President waging war without congressional approval. That sums up the whole thing,” the New York Democrat said at a news conference following the final vote. Schumer added that “senators worked together to assert Congress’ authority and serve as the check of an overreaching executive branch.”

In contrast, co-sponsor Lee had argued a day earlier that the push to pass the measure was “entirely consistent” with the President’s foreign policy agenda and objectives.

Senate Majority Leader Mitch McConnell of Kentucky

“I support what the President is doing with our foreign policy,” Lee said at a news conference with Kaine and other supporters of the measure. “For me, this is about supporting President Trump in his foreign policy, in his effort to make sure that we don’t get involved too easily, too quickly, in an unconstitutional way, in any war. This is entirely consistent with his policy.”

At the same news conference, Kaine outlined a careful argument of his own, emphasizing at one point that the resolution was “not really about President Trump.”

“Some view this as an effort to tie President Trump’s hands. It’s not really about President Trump. It’s really not even about the President; it’s about Congress,” Kaine said. “It’s about Congress fully inhabiting our Article I role to declare war and taking that deliberation seriously.”

Lee pointed out that the coalition that came together to support the resolution represented “a broad spectrum of points along the political continuum,” saying that demonstrates how the measure itself is “neither liberal nor conservative. It’s neither Democratic nor Republican.”

It was a public display of a key issue that had been addressed and resolved behind closed doors.

Changes made to build support

The original version of Kaine’s resolution had some direct references to Trump administration actions.

But Paul told Kaine that it could be seen as calling out the President himself – a problem for Republican senators who may want to support the effort. While Kaine said that wasn’t his intention, it was a valuable heads-up, one that Lee, who has long worked on these issues, would expand on a short while later.

Lee framed the issue more broadly, asking Kaine frankly whether the resolution was about Trump or, instead, any president. He then added that it’s not really about the president at all, but instead the role of Congress.

Kaine told Lee he was right. He then agreed to drop the language.

“I had to be open to hearing his concern,” Kaine said. “But he asked the question in a really provocative way that made me realize, you’re right, I don’t need that.”

Republican Sen. Susan Collins of Maine

Collins, before she committed to voting for the resolution, raised concern that the measure is worded in a way that would appear to require US forces to be removed from the Middle East entirely should it be signed into law. She also made clear that maritime issues, like freedom of navigation and maintaining shipping lanes, should be considered and addressed.

“She didn’t say, ‘Make that change and I’ll do it,’ by the way,” Kaine recalled of the process of trying to secure Collins’ support. “She just said you ought to think about making that change because it will make it better.”

Kaine made modifications.

Over the course of weeks, he set up meetings in the offices of GOP senators who might be willing to vote with him, walking through pieces of the resolution and seeking feedback. He didn’t talk about them publicly or telegraph they were occurring, but he made a point of meeting early on with Sen. Lamar Alexander, a retiring Tennessee Republican, and Sen. Jerry Moran, a Kansas Republican who voted for the Yemen war powers resolution. He didn’t know how they would vote until the day before the final vote itself, he told CNN, but he wanted to make his case directly to them. Both would go on to vote for the resolution.

Even on Thursday, after weeks of effort that also included navigating the scheduling difficulty of ensuring Democratic presidential candidates in the chamber were back in town for the crucial final vote, Kaine had to work to defeat amendments that supporters thought would sink the resolution.

The Cotton amendment

The passions behind the overall debate itself burst onto the Senate floor in one amendment specifically, offered by Sen. Tom Cotton, an Arkansas Republican and former Army officer who has been consistently, and vocally, the ideological polar opposite to Kaine and Lee on issues of executive power as it relates to military action.

Cotton’s amendment would have created an exemption for military forces deemed to be engaging in operations targeting foreign designated terrorist groups. Kaine and his fellow supporters worked throughout the morning to ensure its defeat.

Sen. Tom Cotton, an Arkansas Republican

One of those colleagues, Republican Sen. Todd Young of Indiana, could be seen explaining his opposition to Cotton’s amendment directly to Cotton.

A frustrated Cotton at one point could be seen sharply waving off Young as the Indiana Republican offered his rationale, underscoring his disagreement. He then came back to Young to continue the discussion, which appeared to grow heated, with Young overheard telling Cotton sarcastically “You’re smarter than everyone, we all know that, Tom,” and responding at one point that he wasn’t there to “come down and get upbraided by you.”

The exchange, witnessed by a CNN reporter, was a window into just how split lawmakers are on the broader issue of executive power – a split that isn’t going away anytime soon, something Kaine willingly acknowledges.

But over the course of years, the debate has shifted. What was once a vote that few were willing to take, let alone force, has occurred multiple times over the last two years, with supporters including Sens. Bernie Sanders, a Vermont independent, and Chris Murphy, a Connecticut Democrat, leading on the Yemen effort. House Democrats voted in January to repeal the 2002 authorization for the Iraq War, which has been utilized to justify military operations ever since, including, at one point, the Soleimani strike.

Kaine and Young, a former Marine who has become a key partner for the Virginia Democrat on the issues, are pressing forward on legislation to repeal the 1991 and 2002 authorizations for the Gulf and Iraq wars. Efforts to address the seemingly all-encompassing one from 2001, along with a proposed retooling of the War Powers Act itself, are being considered.

When, or even if, a fulsome debate on those issues plays out in earnest on the Senate floor remains an open question.

“It’s a difficult needle to thread,” Sen. Jim Risch, an Idaho Republican who’s the chairman of the Senate Foreign Relations Committee, on which Kaine sits, told CNN of the balance between congressional and executive authority. Risch sharply opposed Kaine’s resolution, and has been critical of the prospect of any effort to rewrite or repeal existing military force authorizations.

“Everybody thinks that we ought to have a clear (authorization for use of military force), but there isn’t anybody that can put it on paper that that’s acceptable to everyone.”

But the groundwork is being laid, of that – after years of effort by Kaine, Lee and others – there is no question.

“I hope that we’re setting the stage for an honest bipartisan debate when that day comes,” Durbin said.

CNN’s Ted Barrett contributed to this report.