(CNN) At first glance, they seem like an odd couple -- Sen. Bernie Sanders, the progressive and quintessential outsider, and President Joe Biden, moderate politician and political insider.
And yet, the 79-year-old Vermont senator has become a key voice in the Biden administration: called upon, consulted and indispensable in keeping the liberal Democratic flock in line.
After 30 years in Congress and two presidential runs, it's a new experience.
"As somebody who wrote a book called 'Outsider in the House,' yes, it is a strange experience to be having that kind of influence that we have now," Sanders told CNN's Gloria Borger as they sat together in Burlington, Vermont, recently.
The Biden-Sanders connection is not a love story; it's more a marriage of convenience. But as Biden pushes an unprecedented progressive White House agenda, it's crucial. It's also personal. Over the years -- both as colleagues and as political opponents -- the two men have developed a relationship based on mutual respect and trust.
"We have had a good relationship," Sanders told Borger. "He wants to be a champion of working families, and I admire that and respect that."
As a result, Sanders has entered Biden's political tent.
"I can tell you that Bernie Sanders has real influence" in the White House, senior Biden adviser Cedric Richmond told Borger. "Sen. Sanders is respected."
That respect is mutual.
"One of the things that struck me about Joe Biden is the very strong sense of loyalty, which I like and respect," Sanders said.
There is, of course, a history of political disputes between the two. Biden and Sanders have decades of hard-held disagreements, on issues ranging from health care and free college to how to deal with the situation in the Middle East. And that's just a short list.
"He's more conservative than I am, obviously," Sanders says. "But on the other hand, he's not only a smart guy, he is a good politician who has a sense of where people are at and what is possible. And I think he understands that at this particular moment in American history where working families face so many problems, you've got to go big, not small."
Five months into his presidency, Biden has gone big. First up was the $1.9 trillion Covid relief plan -- passed without any GOP support. He followed that up with a $6 trillion budget, a far-reaching plan to protect voting rights, and a massive proposed infrastructure plan, all of which still hang in the balance.
And now, as Biden wrangles with Republicans over the price tag of an infrastructure bill, Sanders, now the chairman of the Senate Budget Committee, is waiting -- and ready -- to use the budget reconciliation process to push spending through with 50 Democratic votes, if he can get them.
"I have very limited patience," he says. "We learned a lesson from the Obama years. And that is, Republicans will talk and talk, 'We want to work with you. Bipartisan.' Month after month after month, nothing happens."
Sanders has already gone to bat for Biden. First up was the American Rescue Plan, which included direct payments to millions of Americans. When some House progressives considered withholding their votes after a minimum wage increase was taken out, Sanders convinced them to stay on board.
What's more, he publicly touted the bill -- much to the relief of the White House -- calling it "the most significant legislation for working people in decades" in a CNN interview after it was passed.
He told Borger, "Was it everything we wanted? No. Was it a major step forward for the working class of this country? You bet it was."
Biden's shift from centrist to big spender has left progressives pleasantly surprised. Sanders told Borger, "I think the Biden of today is not what I or others would have expected" based on the Biden of 20 years ago.
The shift may have been gradual during the Democratic primaries, but after the election -- and certainly after the January 6 insurrection -- Biden evolved.
Two things, says Sanders, changed Biden's MO: the coronavirus pandemic and former President Donald Trump.
"Covid exacerbated all of the existing problems, in terms of the struggles of working families," he said.
And after Trump's term in office -- and the insurrection in particular -- the stakes grew. Political decisions are not just about dollars, both men agree, but about saving democracy itself. "What Biden sees out there is that if we do not move aggressively, and make it clear to people that government can work for them, then we stand a real chance of losing democracy in this country."
Sanders' impatience also revolves around a political fact -- that, for now, Democrats control the House, Senate and Presidency. The close margins in Congress means there are no votes to spare, so Sanders is looking for common ground, even with moderates, who would be needed to pass any large spending bill under budget reconciliation, a process that requires 50 votes.
"You can vote no on every single bill and say, 'Look, this is not perfect' or 'I disagree. No, no, no,' " the budget committee chairman said. "But that's not my job."
But when asked how he deals with moderates like Sens. Joe Manchin and Kyrsten Sinema, Sanders told Borger, "In all honesty, Chuck Schumer does more of that talking than I do."
As campaign rivals in the 2020 Democratic presidential primary, Biden and Sanders kept things mostly civil. Sanders told his team that he didn't want to launch any personal attacks against his opponent. And when a Sanders surrogate wrote an op-ed that said Biden had a "corruption problem," Sanders both privately and publicly rebuked it.
When Sanders was getting ready to drop out of the race, on a plane back to Burlington after canceling a campaign rally due to Covid, he asked adviser Faiz Shakir to reach out to Biden's team.
"He said, 'Why don't you reach out to them and see if they see a role for progressives in their campaign?' " Shakir said. The Biden team was immediately receptive.
Sanders says they made him feel "very welcome." (Notably, that's unlike Hillary Clinton's team in 2016, when he says he was simply "tolerated" after dropping out.)
This time, with Biden, Sanders suggested that they unite on a series of policy task forces and proposals.
"He wanted to make sure that Bernie and his supporters had a real voice in where we go," Richmond, who served as Biden's campaign co-chair, told Borger.
The Biden team, Richmond says, understood the power of Sanders' supporters. But it was about just more than votes, he adds: "I think that there's some just places where they really align."
Richmond points to climate change, lowering drug prices and raising the minimum wage as areas of common ground.
"Sometimes you just have to remind people that their philosophies are not that far apart," he said.
It doesn't hurt that Sanders and Biden have known each other for years and that they both come from working class families that struggled.
Not surprisingly, they also share an admiration for President Franklin D. Roosevelt, whose portrait now hangs in the Oval Office. Sanders approves of Biden's art selection wholeheartedly.
"I think that was a very, very good sign," he said.
Sanders points to one defining moment in this relationship. When Biden decided not to run for president in the 2016 race, and the Democratic Party establishment lined up behind Clinton, Biden invited Sanders, Clinton's rival, to meet with him at the vice president's residence.
"He was giving me his advice, political advice," Sanders says of those meetings. "They were, I think for me, very useful conversations and friendly conversations."
Sanders adviser Shakir puts it more bluntly.
"I certainly believe that Sen. Sanders left that meeting feeling that Joe Biden was giving him a, 'Hey, go make your case, Bernie, because there's a lot of people who need to hear it,' " he said.
At a time when Clinton was the prohibitive favorite -- and few in the Democratic Party expressed support for Sanders -- Biden made it clear he wanted the senator to be heard.
This is not to say they will always agree -- they won't. Sanders makes it clear he will continue to push back when he feels it's necessary.
"He does things sometimes that I think are really not a good idea, but I understand why he does it," Sanders said. "Because he's made promises to people, and he wants to keep his promises."