Three years after a bitter face-off with Hillary Clinton, Sen. Bernie Sanders still sees her political machine working against him.

Clinton may not be running for president, but the world of staffers and institutions that buoyed her presidential run, Sanders says, has a new goal: to quash his progressive vision for America.

Over the weekend, Sanders sent the Center of American Progress — a powerful Democratic think tank that’s closely aligned with the Clintons — a scathing letter, hitting several negative articles and videos published by the group’s media arm, ThinkProgress, that he says will make it easier for President Trump to win in 2020.

“I and other Democratic candidates are running campaigns based on principles and ideas and not engaging in mudslinging or personal attacks on each other,” Sanders wrote in the letter, first obtained by the New York Times. “Meanwhile, the Center for American Progress is using its resources to smear Senator [Cory] Booker, Senator [Elizabeth] Warren, and myself, among others. This is hardly the way to build unity, or to win the general election.”

Now some of the most prominent faces in Clinton’s world, like David Brock, who founded Media Matters for America, are already talking about launching an anti-Sanders campaign before the senator’s presidential bid gets too far ahead, the New York Times reported.

Sanders’s campaign sent out fundraising emails on both the letter to CAP and Brock’s proposed anti-Sanders movement.

That there’s animosity between Clinton’s orbit and Sanders’s is old news. But these latest punches show how the dynamics in the party have shifted. CAP, Brock, and the Clinton world have held power over the establishment for a generation; they’ve filled key staffing roles, crafted agendas, and chosen leaders. Sanders has largely been on the outside. Now he’s the Democratic frontrunner for president in a primary that has largely adopted his ideas.

Hillary Clinton, accompanied by her aide John Podesta, who founded CAP in 2003.

Alex Wong/Getty Images

“This is the first race since 1992 that there has not been a Clinton, a Clinton message, or a Clinton presence,” said Anita Dunn, a Democratic strategist with SKDKnickerbocker who served in the Obama White House. “That is a sea change.”

Sanders isn’t fighting with the Democratic Party anymore. Instead, in his bid to lead it, he’s defining his foil: what he sees as a corporatist wing that’s held an outsize grip on the policies and people who run the show in Washington.

Clinton’s world and the “Democratic establishment”

There are a lot of wrinkles to this story.

First, there’s personal grousing between former coworkers turned campaign adversaries. A number of Sanders advisers took a turn through CAP — including Faiz Shakir, Sanders’s campaign manager who helped found ThinkProgress and served as the publication’s editor-in-chief before leaving the organization in 2012.

The animus is well documented; in emails disseminated by WikiLeaks, CAP’s president Neera Tanden called Shakir a “fucker,” and referred to him and a circle of left-wing bloggers as “freaks after Hillary” in an exchange with top Clinton aide and CAP founder John Podesta in 2016. In another 2015 email, Tanden mused that Shakir “hates me.”

To some, that’s all this is: inside baseball and bitter feuding. This isn’t a fight playing out in the grassroots like it was in 2016.

“They all know each other; they all make sure each other is being taken care of financially,” Ray Buckley, the chair of the New Hampshire Democratic Party, said. “We use the word establishment because people can grasp what it is. It’s not the establishment versus non-establishment. It’s everyone in the Beltway and everyone not in the Beltway.”

That said, of the institutions in the Washington beltway that have come to define the Democratic “establishment” for a generation, Center for American Progress is one of the most prominent and powerful.

When John Podesta founded CAP in 2003, he billed it as a “think tank on steroids” to compete with conservative groups like the Heritage Foundation and the American Enterprise Institute. Podesta, who served as Bill Clinton’s chief of staff, counselor to Obama, and campaign adviser to Hillary Clinton, is one of the most prolific establishment Democratic operatives in the game. Tanden, who became CAP’s president in 2011, comes from the same circle; she’s advised Obama and both Clintons. They have built an organization reflective of their own stature that fundraises in the way the Clintons did.

CAP takes money from big tech, big banks, and from 2016 through last year it took $2.5 million from the United Arab Emirates to fund its foreign policy initiative, the New York Times first reported (they stopped taking UAE money in December 2018). As of 2018, CAP reported that corporate funding made up 2 percent of its $60 million annual budget; foreign governments accounted for less than 1 percent.

Israeli Prime Minister Benjamin Netanyahu participates in a conversation at the Center for American Progress with CAP president Neera Tanden.

Alex Wong/Getty Images

It has been a major resource to Democratic leaders, staffing administrations and campaigns. CAP staffers served in the Obama administration and for congressional party leaders. Even Shakir, for example, left CAP to work for Nancy Pelosi and Harry Reid. During the Obama years, CAP was described as a “revolving door” to the administration.

And CAP researchers have shaped the Democratic Party’s mainstream policy agenda for more than a decade. That’s where Sanders’s gripes come in; there have been times CAP has come under fire from the left, like an event with Israel’s Benjamin Netanyahu, which the left saw as serving organizational interests. CAP has rejected that: “The Netanyahu event was arranged with the public and private support of the Obama Administration, and the notion that it was done at the behest of any donor is preposterous.”

In the age of Trump, CAP has joined the Resistance. Their political arm has not endorsed a primary candidate in 2020, and Tanden maintains that her primary goal is to elect a Democrat to the White House. Sanders’ chief of staff, Ari Rabin-Havt said their campaign understands this to be the case, as well.

“I think there is a complete understanding that CAP’s goal is to defeat Donald Trump in 2020 and they are working to do that and in that goal we are completely united,” Rabin-Havt told Vox.

In his letter, however, Sanders is questioning what kind of Democrat.

The feud

To Sanders’s team, a video ThinkProgress published on April 10 suggesting Sanders had backed off from “millionaires” in his “millionaires and billionaires” campaign rhetoric since becoming a millionaire himself was as good as a Trump attack ad; they saw another piece that questioned Sen. Cory Booker’s leftward shift on prescription drug pricing as an assault on progressivism. Sanders said CAP was attacking progressives and their ideas. (ThinkProgress, for what its worth, maintains editorial independence from CAP. While that hasn’t always been the case in the past, writers, in large part because of a 2015 unionization effort, say they have cemented a stronger culture of independence.)

Vox requested an interview with Tanden but was referred to her statement calling the ThinkProgress video on Sanders “overly harsh,” adding that it “does not reflect our approach to a constructive debate on the issues.”

But Sanders isn’t just mad about negative press. He’s questioning “corporate money” influence on the different policy preferences in the party. In effect, his critique of CAP is actually a continuation of his most salient critique of Clinton: The system is rigged for the elite.

“When [Sanders] says establishment, he is saying the elite, the elite conversation, people who have been dictating the mechanics, and the deals that get done,” Shakir told Vox. “They drive the party, they also drive the policy. ... Some of the things Sanders cares deeply about — on trade, on a just foreign policy — CAP has different postures on. There is some concern: [Are corporate donors] a lot of the reason that [the Democratic Party is] not as progressive on some issues?”

There are clear ideological divides between Sanders and Clintonworld. Take health care policy: CAP proposes Medicare Extra For All, which, despite having a similar name to Sanders’s Medicare for All, firmly rejects abolishing private insurance. Instead, it proposes a Medicare-based public option. And Sanders’s health care policy team and CAP’s health care policy team do not get along.

The same goes for trade, where the Sanders ethos is much more restrictionist, and on foreign policy, where it is more anti-interventionist. CAP has moved toward Sanders on other policies, like $15 minimum wage, which they did not support in 2016, but do now.

But there is also a strategic logic to Sanders picking a fight with CAP.

Can Sanders be an anti-establishment frontrunner?

Sen. Bernie Sanders (I-VT), Speaker of the House Nancy Pelosi, and Senate Minority Leader Chuck Schumer attend an event to introduce the Raise the Wage Act for a $15 minimum wage.

Chip Somodevilla/Getty Images

Sanders’s relationship with the Democratic Party has shifted dramatically since 2016. The Democratic Party changed its rules in direct response to Sanders’s concerns, limiting the power of “superdelegates” — the party insiders allowed to vote for whoever they want — in the 2020 primary, preventing them from voting on the first ballot in a contested national convention. He’s found himself a spot on the Senate’s Democratic leadership team as the “chairman of outreach.”

Sanders has the 2020 primary’s biggest fundraising list, the most money in the bank of any Democratic candidate in the field, and a lead in the polls among announced candidates.

But success carries its own challenges. In 2016, Sanders ran as an insurgent against a hidebound Democratic Party. That’s the political style he’s most comfortable with. As he vies for leadership of that party, he needs a new foil, one with enough power to energize his supporters, but not so much popularity that it alienates voters.

CAP, still run by Clinton allies, is a perfect proxy, and Sanders’s campaign manager Shakir announced a 48-hour fundraising drive to “to combat the potential anti-Sanders campaign” Brock teased in the New York Times.

“Here comes the kitchen sink. According to the New York Times, the financial establishment of this country is gathering at ‘canapé-filled fund-raisers,’ plotting campaigns to stop us,” Shakir said in a campaign fundraising email. “David Brock, who led a multi-million dollar smear campaign against us in 2016, is looking to lead the effort and hopes ‘an anti-Sanders campaign’ will start ‘sooner rather than later.’”

Another email to supporters said “CAP’s leadership has been pretty upfront about their disdain for Bernie — and all of us” and that the organization’s “real goal” was to “stop Medicare for All and our progressive agenda.”

Sanders’s rallying cry is that together, the American people will take on pharmaceutical companies, big banks, political party bosses, and despotic world leaders. But for Sanders, the “them” in his “us against them” rhetoric used to be a lot bigger than it is today. He’s less of an outsider in the halls of Congress than he was even three years ago. He’s made nice with Democratic leadership — even Senate Minority Leader Chuck Schumer, with whom he co-sponsored a bill in opposition to Republicans’ tax reform package.

The Democratic National Committee, which Sanders hit relentlessly for propping up Clinton in the 2016 election, is now in regular communication with his campaign.

So for Sanders, the “them” is now institutions like CAP. The “them” is the Clinton allies that serve as a proxy for Sanders’s assault on the corporate wing of the Democratic Party, and whom his supporters are still furious with from 2016.

“It’s on brand,” Shakir said.

Clarification: A previous version of this article cited a stolen Wikileaks email between Tanden and Podesta reflecting on the controversy around CAP’s event with Netanyahu. That email did not directly reference CAP’s financial interests — rather that a CAP board member was happy with the event, and the left was not.