Sen. Bernie Sanders (I-Vt.) once told a fellow left-wing activist that the Democratic Party was too “intellectually bankrupt” to allow the progressive movement to flourish within it.

In a 1985 letter newly obtained by HuffPost in which Sanders debated running for governor, he wrote: “Whether I run for governor or not is really not important. What would be a tragedy, however, is for people with a radical vision to fall into the pathetic camp of the intellectually bankrupt Democratic Party.”

Times have changed.

Sanders, who has served as an independent in Congress — first in the House and now in the Senate — since 1991, is now among the leading Democratic candidates for president, second behind former Vice President Joe Biden in national polls, and tied for first in Iowa. He’s raised more money than any other candidate in the Democratic primary, with more individual contributions. His platform — which includes “Medicare for All,” tuition-free college, and wealth redistribution through aggressive taxes on the richest Americans — defines the party’s progressive wing.

He has taken the Democratic Party’s loyalty pledge and repeatedly promised to support whoever becomes the party nominee to defeat President Donald Trump. But that hasn’t been enough for many of his critics in the party’s establishment, who are likely to point to the letter as evidence he doesn’t play nice with the party.

At the start of his career in politics, Sanders viewed the Democratic Party as hopeless. By 1985, he had already made several failed bids for elected office in Vermont as a member of the progressive Liberty Union party. He finally succeeded in 1981, becoming mayor of Burlington, the state’s largest city, and toppling a five-term Democratic mayor.

“At the beginning, the Democrats were very angry with him,” Jane Sanders, Sanders’ wife, said in an interview with HuffPost. Burlington’s Board of Aldermen — a part-time, 13-seat city council — resisted Sanders’ efforts. “They wouldn’t let him appoint any of the mayoral appointments. They stopped him from hiring the city attorney, city clerk, city treasure ... [The Democrats] were outright enemies in the beginning.”

Sanders never won full control of city government, but he organized enough voters to elect some allies onto the board and win veto power. He soon became popular enough to win reelection by a 20-point margin.

The Sept. 5, 1985 letter ― which has not previously been reported on ― came as Sanders was considering a run for governor, and left-wing activists in the state and around the country were debating whether he should mount an independent bid or launch a primary challenge to the state’s Democratic governor, Madeleine Kunin.

Obtained by HuffPost A Sept. 5, 1985, letter from then-Burlington, Vermont, Mayor Bernie Sanders to Marty Jezer, a progressive activist.

Sanders’ three-paragraph missive was addressed to Marty Jezer, an author and progressive activist in the state. Then-Mayor Sanders was writing in response to an August letter from Jezer in which he apologized that a memo he wrote to Sanders had leaked to the press. While the exact contents of the memo are unclear, Jezer’s letter indicates that it encouraged Sanders to run for Congress instead of challenging Kunin.

“1986 is the wrong time for such a race,” Jezer, who died in 2005, wrote. “I hope you will listen to the voices of the committed activists around the state. We sink or swim with this together.”

Sanders ultimately reached a different conclusion: He ran against Kunin as an independent. But the decision was not without dissent. An editorial from the socialist magazine In These Times criticized Sanders for dividing the left.

“In choosing to create a three-way race, Sanders is dividing the left and making more likely the defeat of an incumbent liberal woman governor by a more conservative Republican,” In These Times wrote. (At the time, Kunin was one of only two female governors in the country.)

The editorial prompted Sanders to reply: “I believe that the real changes that are needed in this country … are not going to be brought about by working within the Democratic Party or the Republican Party.”

Kunin eventually won reelection with 47% of the vote. Republican Peter Smith finished second, earning 38% of the vote, while Sanders finished third with just 14%.

The Vermont senator’s critiques of the Democratic Party are well documented, as CNN reported last July. Throughout the 1980s and 1990s, he was adamant that a progressive movement could not be built within the party and was highly critical of the moderate “New Democrats” who argued that the party’s progressivism in the 1960s, 1970s and early 1980s had alienated voters.

“I think that nationally, the party has on issue after issue sold out so many times that if you go before the people and say, ‘Hey, I’m a Democrat,’ you don’t usually generate a lot of enthusiasm,” Sanders said in 1991 about the idea of a progressive trying to work within the party.

Commenting on civil rights activist Jesse Jackson’s Democratic presidential runs in the 1980s, Sanders said he did not agree with Jackson’s decision to work “within the Democratic Party.” (Sanders endorsed Jackson’s candidacy.) His skepticism of the party continued in subsequent decades. In 2011, he said Democrats could be called “Republican-lite” for considering cuts to Social Security and Medicare in order to lessen the deficit. And his first presidential campaign in 2016 didn’t shy away from blasting the party apparatus.

Sanders’ willingness to criticize the Democratic Party speaks to the progressive bona fides highlighted by his supporters. His campaign often relies on decades-old videos of Sanders warning against the Iraq war, multinational trade deals and the climate crisis using the same rhetoric he still uses today.

But the senator’s view of the party — and the role of progressive politics within it — has evolved. He’s since refined his critiques to focus on the “corporate wing of the Democratic Party,” which is composed of the same centrists, including organizations like Third Way, that pushed the party to the right during the 1980s and ’90s.

Jane Sanders, who did not recall the exact circumstances of her husband’s letter to Jezer, said this shift came about when he was elected to the House as an independent. He knew that if Democrats didn’t let him into their caucus, he wouldn’t be assigned to any committees and would be left out of the process altogether.

“He believed that the two-party system was bankrupt, but as he was put in a position by the people of Vermont to effect real change, he had to consider, how do you effect real change?” Jane Sanders said. “His concern isn’t party politics.”

By 2015, when he launched his bid for the Democratic presidential nomination, Sanders had fully changed his mind about the ability for progressive politics to flourish within the party.

“The decision that I made, as the longest-serving independent in the history of the United States Congress, I said, ‘If we are going to win this race, we have to do it within the Democratic primary process,’” Sanders said in a 2016 interview with NBC’s Chuck Todd. He decided to not run as an independent because he didn’t want to be a “spoiler” in the presidential race.

“Bernie fights when he has to fight,” Jane Sanders said. “Bernie’s interest is getting things he wants done, done. He found that he had to fight effectively against the Democratic Party at the beginning as mayor. But over time, by organizing, he learned he can change the Democratic Party.”

Sanders’ relationship with the party has notably softened since that first presidential run. He served on the Senate Democrats’ leadership team and his 2016 campaign managed to push the Democratic National Committee to change rules around “superdelegates,” lessening the power of party insiders in the nominating process.

That hasn’t been enough for many of his critics, who accuse him of only half-heartedly campaigning for former Secretary of State Hillary Clinton in 2016 after dragging out the primary, and question whether he would be willing to support down-ballot Democratic candidates who don’t share his progressive ideology.

Jane Sanders said she hadn’t spoken with her husband about how he would engage in party politics should he be elected president. But she was explicit about how he approaches it currently.

“If you organize, the people will elect new people,” Jane Sanders said, citing those like Rep. Alexandria Ocasio-Cortez (D-N.Y.), who famously ousted a top-ranking House Democrat in the 2018 midterm elections. “You will see Bernie supporting and raising money for the best of the Democrats. You will not see him doing that for others.”

But former Vermont Gov. Peter Shumlin, a Democrat who has endorsed former Vice President Joe Biden in the 2020 nominating contest, said he was skeptical that Sanders could keep the party together during an election against Trump.

“The notion that [Sanders] would lead the national Democratic Party would be repugnant to both Democrats and to Bernie Sanders, because he’s not a Democrat,” Shumlin told HuffPost in a phone interview. “He’d be the first one to tell you that.”

That the party’s policy debates have only now caught up to him was Sanders’ opening pitch to voters when he announced in early 2019 that he would run for president again.

“Over the last two years — and before — you and I and millions of Americans have stood up and fought for justice in every part of our society, and we’ve had some successes,” Sanders said when launching his 2020 bid.

As one of the top contenders going into the Iowa caucuses in early February, Sanders now clearly believes that he can bring about a progressive revolution within the Democratic Party — one that he’s worked to change.