CHARLESTON, S.C. — Bernie Sanders was threatening to kill Obamacare.

It was December 2009, and the iconoclastic senator had mounted a rebellion against Democratic Senate leaders who were moving to scrap the "public option" to make the bill more palatable to two conservative Democrats. The party had 60 available votes and needed every last one to successfully cap a century of effort to remake health care.

Sanders went on MSNBC's "The Ed Show" and didn't mince his words: "I'm threatening not to vote for it." He said a lack of "a strong public option" to compete with private plans and control premiums would mean it "ain't a strong bill — that's a pretty weak bill."

Two weeks later, Sanders voted for the bill despite the removal of the public option, conceding that it was "not as strong as I wanted" but "it begins to move this country toward the long-time goal of providing comprehensive, affordable health care for all Americans."

The episode captures a widely misunderstood feature of Sanders' three-decade legislative career: He has repeatedly, if grudgingly, supported compromises that fall short of his ideal as a self-described democratic socialist.

That may surprise voters in 2020 who see an unyielding purist leading the Democratic presidential field with calls for a "political revolution." Here was a bill that rejected Sanders' preferred single-payer system and entrenched the private insurance market he wants to eliminate. Yet he voted for it to advance the goal of expanding coverage after securing some funds for community health centers.

"We felt — correctly in the end — that he would come around and not stand in the way of trying to pass it," said Jim Manley, who was then the spokesman for Senate Majority Leader Harry Reid, D-Nev. "In my years in the Senate, the leadership spent far more time worrying about the more conservative members of the caucus than Senator Sanders."

Months later, Sanders voted for sweeping Wall Street regulation that excluded two of his top priorities — breaking up "too big to fail" banks and reimposing the Glass-Steagall law. In 2014, when Democrats had made Sanders chairman of the Veterans Affairs committee, he wrote a compromise with Sen. John McCain, R-Ariz., and the House Republican chairman Jeff Miller, R-Fla., to overhaul a VA system in crisis.

Manley, who made clear he doesn't want Sanders to be the Democratic nominee, said the criticism of him as an inflexible socialist "doesn't bear any relationship to his legislative activity in the Senate."

"He is not the Senate version of the so-called House Freedom Caucus, where you have a bunch of nihilists willing to tear the place down," he said. "He likes to draw lines in the sand and take tough positions, but like most Democrats he believes in governing. He isn't there to blow things up."

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Sanders has, however, shown more of a penchant for purity on trade deals. He opposed presidents of his party on the North America Free Trade Agreement and the Trans-Pacific Partnership, and recently voted against changes to NAFTA that most Democrats, including Sen. Elizabeth Warren of Massachusetts, deemed beneficial to workers. He has boasted about opposing measures that are now unpopular with liberal voters, including the bank bailout of 2008, the Iraq war and the bankruptcy bill.

The image of purity is a double-edged sword. It benefits Sanders with liberal and disaffected voters who admire his moral clarity and conviction, but it has left him vulnerable to criticism that he'd be a feckless president wedded to an agenda that's going nowhere.

On NBC's "Meet the Press" last week, Biden said Sanders has "never gotten anything done" and called it "fanciful" and dishonest for him to run on unrealistic proposals like "Medicare for All."

"I think people are so tired of the lack of straightforwardness out there. Now, if I'm wrong, I'm going to be dead wrong, but I really believe that you have to lay out why you're doing what you're doing and how you're going to get it done," Biden said.

At the Las Vegas debate last week, Pete Buttigieg urged the Democratic Party not to nominate "somebody like Senator Sanders who wants to burn the house down."

The Sanders campaign is reluctant to rebuff the critique of him as a rugged ideologue, a move that would risk turning off disaffected voters who are attracted to that side of him. Unlike rivals like Biden and Buttigieg, Sanders believes campaigns are less about talking to voters about what is politically realistic and more about trying to reshape it, say people familiar with his thinking. He told a crowd Wednesday in North Charleston, South Carolina, that his revolution means bringing "young people into the political process in a way that we have never seen in the history of this country."

Top Sanders adviser Jeff Weaver recently said that it's "premature" to say whether Sanders would support a public option in the event that Congress rejected a single-payer system.

"What he does believe is you don't start off compromising with yourself, as sometimes happened," said Michael Briggs, a former Sanders aide in his Senate office and his 2016 presidential campaign.

Briggs said that Sanders' approach in the Affordable Care Act debate is "very illuminating" about his governing philosophy. "It shows he has set down markers for what he believes should be done, but is also willing to accept progress toward his goals when it becomes clear that that compromise is what's going to be doable."

Warren, his progressive rival, has said she'd begin her presidency by setting up a public option and push for Medicare for All later. She has argued she'd be a more "effective" president than Sanders because she's steeped in policy details and is willing to support initiatives like removing the legislative filibuster to pass legislation, which the Vermont senator has not endorsed.

One former Republican senator who worked with Sanders said that they had worked well together and that Sanders "was willing to accept half a loaf" until he began running for president in 2015. "So I never saw that side of Bernie again," he said. "The caricature and stereotype become more true to form."

When speaking to voters, Sanders' prescription for complex problems tends to revolve around defeating a clear enemy — health care industry profits stand in the way of Medicare for All, and fossil fuel earnings are preventing progress on climate change. Where rivals see cringe-worthy simplicity, his supporters see a politician who tells it like it is.

"He doesn't talk about these issues as a legislator. He talks about them as a leader," said Tad Devine, who was chief strategist for Sanders' 2016 campaign. "He's addressing this in the context of electoral politics where people are either inspired or bored."

Sanders argued that the seeds of his movement are bearing fruit.

"I'm very proud of my record of accomplishments," Sanders told NBC News. "As a result of some of the work that our campaign has done, we've seen the minimum wage going up to 15 bucks an hour all across this country. We have pumped billions and billions of dollars into community health centers all over this country. I helped along with John McCain to write one of the most significant pieces of veterans' legislation in the modern history of this country."

All three items he listed were the products of compromise.