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WASHINGTON — It seemed like a surprising party of two.

There was Robby Mook, Hillary Clinton’s top campaign aide, known for his calm temperament and fiercely disciplined ways, and Jeff Weaver, a combative political fighter often called Bernie Sanders’ alter ego, sharing a Friday night dinner at The Farmhouse Tap & Grill in Burlington, Vermont.

But over the long months of a frequently contentious primary, the two rival Democratic campaign managers struck up an unusually friendly relationship, founded on exhaustion, goofy jokes and a shared affection for their home state of Vermont.

They talk almost daily, text frequently and email often.

Now, as Sanders lingers in the presidential race, refusing to concede the nomination to Clinton even as he says he’ll vote for her on Election Day, the competing campaign managers have become a powerful political odd couple, responsible for engineering a graceful conclusion to a hard-fought Democratic contest.

“I’ve really come to respect him,” Mook said. “There were some tense moments, but he was always honest, straightforward and very easy to work with.”

Weaver is equally effusive in his praise.

“I think he’s the kind of guy who is doing what he does for the right reasons,” Weaver said about Mook. “He believes in the cause and he believes in making the world a better place.”

After Clinton and Sanders met at Washington hotel this month, their managers stayed until almost midnight, attempting to hammer out an agreement that would give Sanders some of the changes he wants to make to the party’s platform. During his Friday trip to Vermont, Mook made sure to meet with Sanders supporters.

Some of the communication hints at far closer cooperation to come.

The two camps are increasingly comparing notes on how best to attack presumptive GOP nominee Donald Trump. Clinton’s campaign and state Democratic parties have hired some Sanders staffers, and there is chatter about joint events to come.

Both Mook and Weaver share a slightly silly sense of humor.

Mook, 35, regales his fiercely loyal band of young operatives, known as the Mook Mafia, with impressions, including spot-on impersonations of Bill Clinton and Sanders.

Weaver, 50, who owns and operated a Falls Church, Virginia, comic book and gaming store before taking the helm of Sanders’ campaign, made up gag business cards at the start of the campaign describing himself as the “comic book king.”

“His Bill Clinton is pretty good,” Weaver said of Mook. “It’s not only the voice, but it’s the subject matter.”

But their back-channel negotiations are nothing but serious.

While Clinton has largely unified Democratic leadership around her bid, she’s struggling to win over the young and liberal voters who supported Sanders, a Vermont senator.

Sanders is pushing for ways of addressing key economic issues in the Democratic platform, including trade, providing free college tuition and expanding Medicare and Social Security.

“Right now, what we are doing is trying to say to the Clinton campaign, stand up, be bolder than you have been. And then many of those voters in fact may come on board,” Sanders told CNN’s “State of the Union” on Sunday.

He also wants procedural changes, such as allowing independents to participate in primaries and curtailing the role of superdelegates — the party leaders who help determine the party’s nominee.

On Friday, Sanders told MSNBC that he would vote for the former secretary of state. But he shied away from offering a formal endorsement or urging his supporters to back her.

Instead, he’s kicked off a new phase of his “political revolution,” campaigning on behalf of like-minded Democrats who are running for Congress or local office.

To close that gap, the candidates may rely on the personal rapport between their two top aides, a relationship helped along by formative years in Vermont politics.

Weaver was raised in a rural, northern Vermont town. Mook, the son of a Dartmouth professor, grew up in Norwich, near the New Hampshire border.

As a 20-year-old Boston University student, Weaver drove Sanders around the small state during Sanders’ unsuccessful campaign for governor. Mook’s first campaign memory: going to the dump to get petition signatures and distribute literature.

While they knew of each other, the first time they met in person was in October, at the Iowa Democratic Party’s Jefferson-Jackson Dinner, a key stop for presidential candidates.

Wearing matching outfits of khakis, blue blazers and Johnston & Murphy brown shoes, they posed for photos with their legs propped up on a security barrier.

“His shoes were in better condition,” joked Mook.

In New Hampshire, they were subjected to a series of interviews about each other’s campaigns — while sitting kitty-corner. The experience was remarkably friendly, Weaver recalled, allowing them to commiserate over the lack of sleep and endless travel that is part of a presidential campaign.

After that, the conversation slowly expanded. Today, their relationship has grown far closer than that of their bosses.

Though Clinton and Sanders have known each other since she came to Washington as first lady in 1993, they rarely communicate, say aides.

Former President Bill Clinton, according to aides, was particularly frustrated by Sanders’ ability to cast himself as above politics-as-usual while firing off what he considered to be misleading attacks on Clinton’s White House legacy.

For Weaver, his focus remains on ensuring that Sanders and his supporters are represented in the party and the platform that will be voted on at the Philadelphia convention.

“It obviously is important that the secretary during the general election speaks to the aspirations of that 13 million people who voted for Bernie Sanders,” Weaver said. “It’s important those people be heard — not just feel like they’ve been heard — but be heard.”