(CNN) —  

The unexpected decision by Senate Democratic Leader Harry Reid to retire has prompted a surprising and divisive fight among two of the top Senate Democratic leaders and is raising doubts as to whether one of them, Sen. Dick Durbin of Illinois, will be able to retain his leadership post.

The dispute between Durbin and Sen. Chuck Schumer of New York is centered on whether the two men agreed to endorse each other for their leadership positions in the next Congress -— that begins in two years — after Reid retires.

The conflict comes after Reid had promised an orderly transition. It has the potential to fracture the Democratic caucus between senators loyal to one man or the other, at a time when Democrats desperately need unity in their legislative and political battles with the Republicans who control the chamber for the first time in eight years.

READ: Harry Reid wouldn’t ‘let the bastards beat me’

Durbin is currently Reid’s No. 2, and he wants to keep that job. Schumer is currently Reid’s No.3, but ihe s poised to leap-frog Durbin to take over the top job in January 2017. That’s because Reid, who is very influential with the Democratic senators, endorsed Schumer over Durbin immediately after he announced last Friday that he would not run for re-election.

Sen. Patty Murray of Washington, currently the No. 4 elected Democratic leader, is widely expected to move up at least to the No. 3 job. But as the fisticuffs between Durbin and Schumer spill into public view, she is quietly waiting in the wings to see if there will be an opening for her to move even higher.

At issue is what happened about 2 a.m. last Friday, when the Senate was wrapping up nearly 16 hours of voting on amendments to the annual budget bill. Durbin and Schumer huddled privately off the Senate floor to discuss the revelation that Reid would step down.

Both sides agree that Durbin vowed to endorse Schumer to replace Reid, telling him in an emotional moment, “You’ve earned this.”

But the two camps disagree on what happened next.

As Durbin’s side tells the story, Schumer returned the gesture, saying he would endorse Durbin to stay in the No. 2 job, which is known as the whip.

“The two senators agreed to support one another and shook hands on it,” said Ben Marter, a spokesman for Durbin.

Schumer’s camp disagrees.

SEE: Harry Reid’s bad phone manners

“That didn’t happen, and they know it,” said Matt House, a spokesman for Schumer.

Regardless of what happened that night, Schumer’s refusal to endorse Durbin makes it complicated for the two men to work together. Schumer is known to admire Murray, who has taken on a number of complicated assignments for the caucus — including chairing the budget “super committee” that tried and failed to cut a grand bargain with Republicans on taxes and spending. Moving a woman to the second-ranking position also carries broad political benefits for Democrats.

“Sen. Murray is focused on her current job and isn’t going to be speculating about other positions under Sen. Schumer in leadership elections two years from now,” her spokesman, Eil Zupnick, said.

Staying out of the fray and keeping silent about her intentions also allows Murray to avoid angering either Schumer or Durbin at this delicate time.

A former top Senate Democratic aide, who asked not to be identified to assess caucus politics, predicted Murray would lay low for now.

“I think she will bide her time. See how things play out. In the meantime, she’s assessing the lay of the land to see if she can mount a challenge or not. If a challenge is needed,” the former aide said.

There has been private grumbling in some Democratic quarters that Durbin is not rough and tumble enough to effectively challenge Senate Majority Leader Mitch McConnell, a famously hardball floor tactician. The complaint is that Durbin is an old-school institutionalist who believes in negotiation and compromise at a time that tea party-backed Republicans like Sen. Ted Cruz of Texas and Sen. Rand Paul of Kentucky are dominating the Senate.

“Is that fair criticism? Yeah, that’s fair criticism, but people change, circumstances dictate how leaders act, and I think Durbin would rise to whatever the current challenge was should he be given that role,” said a person close to Durbin who asked not to be identified in order to speak candidly about the situation.

The public friction between Schumer and Durbin is uncharacteristic for the pair. Until last year, they were roommates for more than two decades in a cramped Capitol Hill townhouse. They probably still would be had their landlord, Rep. George Miller, D-California, not retired from Congress. And they have been close friends even longer, having served in the House together since the early 1980s.

They also appear at ease with each other at news conferences, hearings and other events where they regularly espouse similar views on issues of the day and occasionally josh around. In fact, current and former Democratic Senate aides say there’s been surprisingly little backstabbing among the top four Democratic leaders over the several years they have been a leadership team.

However, there has been some simmering tension between Durbin and Schumer for the past five to seven years as both have angled to replace Reid.

Durbin was just re-elected to a new six-year term, and according to a person close to him, he probably won’t run again. The person cited this as one of the reasons he decided against challenging Schumer, believing a leadership campaign would have been a two-year slugfest that would have split and weakened the caucus.

What remains unclear is why Schumer and Durbin have such starkly different views of what happened that night.

Durbin’s side says the men discussed specifics of what their roles would be if Schumer was No. 1 and Durbin No. 2. Durbin would dig into policy matters like negotiating bills and steering legislation on the floor; Schumer would be the visible leader and deal with the political side of the operation, including fundraising and overseeing campaigns.

Further complicating the situation: Durbin’s spokesman says Reid also endorsed Durbin for the whip job. But Reid’s office declined to verify that.

Durbin and Schumer spoke late Wednesday, according to their spokesmen, neither of whom would discuss details of the talk.

There was one small sign Wednesday that Schumer and Durbin might be moving to find a peaceful way forward: A re-issued statement from Schumer’s press aide, denying the senator had endorsed Durbin, carried a softened tone.

“That’s not what happened but regardless, Sen. Schumer considers Sen. Durbin a close personal friend,” the new statement said.