Bernie Sanders did not like to open the phone lines to political critics when he hosted his Friday radio show on WDEV, a shoebox station in Waterbury, Vt. But there was one caller with whom he had no disagreements: Elizabeth Warren, then a Harvard Law professor who would join him for occasional rap sessions about the destruction of the middle class.

“They were kindred spirits, that’s for darn sure,’’ recalled the station’s longtime owner, Ken Squier, of those radio shows back in 2003 and 2004. Said Warren: “We sometimes could finish each other’s sentences.’’

A dozen years later, Warren and Sanders have emerged from the cozy confines of Green Mountain broadcasting to the center of the Democratic Party’s anguished debate over how to blaze a path out of the political wilderness after the election of Donald Trump.

Their history is more fraught and their future more uncertain than their ideological alliances might suggest.

But distraught liberals have turned to the two fist-shaking firebrands as the pair they believe can tap into the populist anger Trump channeled among disaffected working-class voters while also fighting Trump’s attempts to roll back environmental protections and civil rights.

“It’s more true than ever that the leaders of our party and the leaders of our movement are going to be Bernie and Elizabeth,’’ said Charles Chamberlain, executive director of Democracy for America, which tried to draft Warren to run for president before endorsing Sanders. “It’s not going to come from the old Wall Street wing of the party.’’

Still, tensions may lie ahead.

Warren is regarded as more of an inside player in the Senate, and was given a leadership position by Senate minority leader Harry Reid, while Sanders, an independent and self-professed socialist, stands proudly and often defiantly apart from his colleagues.

Both command loyal national followings, are among the Senate’s most prodigious fund-raisers, and will have to decide when to join Democratic leaders who want to negotiate with Trump and when to oppose concessions. Neither is likely to give up the megaphone.

“There really can be only one custodian of the heart of the Democratic base, and I see friction between them,’’ said Ross K. Baker, a Rutgers University political scientist.

So far, Warren and Sanders have signaled similar approaches to Trump’s victory, pledging to join with him if he wants to help workers but fight him if he attacks women, immigrants, or minorities.

The two first met nearly two decades ago when Warren was invited to speak at a dinner party for liberal lawmakers at the Washington home of Representative Rosa DeLauro, a Connecticut Democrat.

Warren, who had not yet published “The Two-Income Trap,’’ the 2003 book that would vault her to national attention, recalled passing out photocopies of her research on the housing market to Sanders, who was then a little-noticed member of the House.

“Bernie started in with the questions,’’ Warren said. “We got into a deep conversation about it. What it meant. Why it was happening. What we could do to help.’’

Over the years, Warren returned several times to DeLauro’s dinner salons, offering her views on the economic forces chipping away at the middle class. Sanders was there every time, she said. Then they talked in his office and by phone.

“It was organic,’’ Warren said of the relationship. “It kind of grew up.’’

The two bonded over Warren’s research that showed how easy it was for middle-class families to go bankrupt if they experience a crisis like a medical emergency or job loss.

“That was the heart of it,’’ Warren said. “That is what Bernie and I talked about. How it was that the American middle class became so vulnerable.’’

On WDEV, where he hosted “The Bernie Sanders Show,’’ Sanders invited Warren to discuss her findings from her home in Cambridge. The two were earnest and in constant agreement.

“Was it wonderful radio?’’ said Squier, the station owner. “I can’t say that.’’

But the relationship became an important one to both of their careers. In 2008, Sanders, then a senator, invited Warren to speak at a series of town hall meetings in Vermont.

At one meeting, in the Montpelier High School cafeteria, he described constituents forced to split wood to heat their homes because of the high cost of heating oil, while Warren displayed bar graphs on a screen that showed how mortgage payments and health insurance costs had devoured paychecks between 1970 and 2000.

Two years later, Sanders urged President Obama to pick Warren to help set up the new Consumer Protection Financial Bureau, and when he did, applauded the president for turning to “a tough and brilliant advocate’’ to ensure Americans would not be “ripped off by large banks and financial institutions.’’

When Warren ran for the Senate against Scott Brown in 2012, Sanders campaigned for her in the towns in the Berkshires that are so much like his home state of Vermont.

“I knew Elizabeth Warren before she was Elizabeth Warren,’’ Sanders told about 200 politicians and union activists at a breakfast event in Pittsfield that year. “I knew her when she was writing books and working as an educator.’’

The two faced their greatest rift earlier this year, however, when Warren refused Sanders’ repeated entreaties to endorse his run for the presidency.

Given their shared values and history, her stance angered many on the left, but Sanders’ aides insist he bears no grudge.

They point out that Warren was the only Democratic woman in the Senate not to endorse Hillary Clinton early in the primary and that she waited until June to back Clinton, after the former secretary of state had effectively secured the nomination.

“The fact that she chose not to endorse until the voting was over is a big deal,’’ said Tad Devine, a Sanders adviser. “That’s something that anyone would appreciate.’’

Last month, in a fence-mending gesture, Warren accepted Sanders’ invitation to join him at a Clinton campaign rally in Colorado. There, she paid tribute to his failed campaign.

“To every person who ‘felt the Bern’ during the primary, America and the Democratic Party know the power and energy of the progressive movement,’’ Warren told the crowd.

Now, as Democrats face a grim and uncertain future under a Trump White House and a Republican-led Congress, Warren and Sanders have been among the most vocal party figures trying to chart a path forward.

Paul Kirk, who was Massachusetts’ interim senator from 2009 to 2010, after Edward M. Kennedy died, said Warren’s voice will be vital as a leading critic of Wall Street. And Sanders’ influence may grow, Kirk said, because his campaign, like Trump’s, showed how much voters want to dismantle the political establishment.

“Senator Sanders arguably has more credibility than before because, for the Democrats, he was the authentic change agent,’’ said Kirk, a former Democratic National Committee chairman who endorsed Sanders in the primary. “His voice, on the Democratic side of the aisle, will have even more resonance.’’

On that point, even detractors agree.

“They’re going to be a voice within the party that has to be listened to,’’ said former senator Judd Gregg, a New Hampshire Republican, who pointed out that Warren and Sanders have pushed for single-payer health care, tax increases on the wealthy, and debt-free college. “They’re carrying the mantle of socialism, and it’s on the rise in the Democratic Party.’’

Michael Levenson can be reached at mlevenson@globe.com. Annie Linskey can be reached at annie.linskey@globe.com.