In seeking the 2020 nomination for the presidency, Sen. Bernie Sanders (I-VT) had made overtures that he’d operate more firmly within the Democratic Party as the party adopted procedural reforms to accommodate his concerns.

The rapprochement was always delicate. And this week it hit a major snag as the senator’s presidential campaign opened fire at one of the Democratic Party’s leading think tanks over a video that its editorially independent news site posted on Sanders’ personal wealth.

That video from ThinkProgress and the senator’s response, in which he accused the Center for American Progress of bias against liberal candidates and veneration for corporate interests, exposed the lingering animus between the Sanders and the Democratic Party’s actual institutions.

It also raised alarm and questions as to whether Sanders was running to lead the party or to fundamentally change it.

“If you always want to be an aggrieved factional candidate, then you do what they did here,” said a Democratic operative who is a fan of Sanders. “It has nothing to do with the electoral context of Iowa or New Hampshire or Nevada or South Carolina. Let’s go fight that battle. Voters don’t care about CAP.”

The root of the latest blow up was a video produced by ThinkProgress noting that Sanders had stopped maligning millionaires—leaving his criticism for billionaires—when he became one himself. The news site is part of the CAP umbrella, which gave the video the veneer of a sanctioned attack. But it also claims editorial independence from the think tank, though the degree of that independence is difficult to define.

Sanders’ campaign was initially uncertain of how it should respond to the post. But a day after it had been up—and shared gleefully by Republican operatives—they chose to push back in a way that, Democrats said, redefined disproportionality.

Over the weekend, Sanders’ campaign sent a letter to the board of CAP and CAP Action Fund saying that the “counterproductive negative campaigning needs to stop.” The letter referenced content written about Sanders and two close colleagues who are also in the 2020 race: Senators Elizabeth Warren (D-MA) and Cory Booker (D-NJ). It also explicitly called out CAP president Neera Tanden, an ally of Hillary Clinton, who has been critical of Sanders in the past but has attempted to mend bridges.

Among some Democrats, there was a sense of bewilderment that the Sanders campaign had gone—as one operative put it—”nuclear” over a mere web video. One Democratic consultant sympathetic to Sanders described the video as being “like a gnat buzzing around your ear,” one which should not have distracted from the candidate’s Midwest tour.

It also raised questions as to whether the senator was ready for the scrutiny that would come from being a frontrunning candidate, after having run as an insurgent against an ideal foil, Hillary Clinton, in 2016.

“When you’re leading in the polls of president of your party you should expect investigative stories to hit at least once a week and to be attacked by your opponents every day,” said Ben Labolt, who served as press secretary to Barack Obama during the 2012 campaign. “An attack on something like ThinkProgress is the sign of a super-narrow-minded campaign that isn’t actually thinking of how the election will be won... They have chosen an establishment force that no one outside of the Starbucks at 16th and K would recognize.”

But within Sanders’ orbit, the pushback was seen as strategically prudent. Sanders has often bristled at personal questions that he deems irrelevant to the set of beliefs he has espoused for decades. And his attack on CAP effectively set a benchmark for the type of coverage that the his team would countenance and reinforced his brand as someone outside of typical party structures. It also undermined any notion that he was a political pushover—a suggestion that lingered for some after he muted some of his attacks on Clinton in 2016.

It didn’t hurt matters that some prominent, though non-establishment, Democratic figures offered Sanders their support. Tom Steyer, the liberal billionaire donor and party activist, who also serves on the board of directors for CAP issued a statement on his own saying he would use his “voice on the Center for American Progress’ Board of Directors to discourage any such attacks on any candidate seeking the Democratic nomination in the future.”

Those close to Steyer told The Daily Beast that he released the statement without consulting with others at the organization and that it by no means suggested he was endorsing Sanders. Another CAP board member, Stacey Abrams, declined to comment. But a spokesperson for the former Democratic gubernatorial candidate in Georgia directed The Daily Beast to a conciliatory statement Tanden issued on Monday afternoon, saying the video had been “overly harsh” and did “not reflect our approach to a constructive debate of the issues.”

That, for now, seems to have quieted the skirmish with Sanders’ campaign manager Faiz Shakir saying that the campaign looked “forward to working in a more productive manner” with CAP, “if possible.” But within a few hours, Sanders was, once more, putting pressure on the press; this time on an outlet of a highly-different ideological bent than ThinkProgress.

In a town hall with Fox News, the senator hit back on the suggestion that his income-inequality message was muddied by his personal wealth by directly challenging the newscasters to ask the president for his own tax rates.

“I pay the taxes that I owe,” he declared, “and by the way, why don’t you get Donald Trump up here and ask them how much he pays in taxes?”