Bernie Sanders won one of the most significant endorsements of his presidential campaign on Thursday when Communications Workers of America, the largest communications and media labor union in the country, announced its support of the Vermont senator’s White House bid. Sanders’s growing support from big labor, however, poses challenges to a candidacy built around his independence from big money and special interests.

National Nurses United and the American Postal Workers Union have previously backed the self-described Democratic socialist. The CWA, which counts 700,000 members, is the largest union to stand in Sanders’s corner so far. Larry Cohen, the union’s former president, acts as a labor advisor to the Sanders campaign.

Union support helps build momentum for Sanders’s White House run, but may  create new headaches for the campaign. That tension was on display at an event announcing the endorsement on Thursday. Sanders criticized the influence of big money in politics and repeated his promise to stay far away from super PACs. But CWA president Chris Shelton indicated the union is likely to use its super PAC to support the candidate.

“We will use whatever we need … to do every single thing we can to get Bernie Sanders elected to the presidency of the United States, so we will use our PAC money,” Shelton said, when asked how CWA planned to use its super PAC to support Sanders. “If Bernie doesn’t want to take it, okay I respect that.”

For his part, Sanders sought to draw a distinction between grassroots political activity organized by labor unions and the power of billionaires to buy elections: “Any comparison about working people knocking on doors as opposed to billionaires making a contribution, I think, would be a false comparison,” he said. “As you know, I have said I do not want a super PAC … I will never raise money for a super PAC.”

The labor union echoed that logic. “We will respect Bernie's wishes but we will use all legal and possible resources to get him elected,” Candice Johnson, a spokesperson for the union said. “We do have a super PAC, but it’s a super PAC of a union of 700,000 working people, not a couple of billionaires. That’s a big difference.”  

Sanders already faces scrutiny over his ties to National Nurses United, which endorsed him in August and has used an affiliated super PAC to spend significantly in support of the senator. Despite Sanders’s forceful insistence that he opposes super PACs, he has appeared hesitant to disavow the effort. It’s one facet of a broader challenge facing the Sanders campaign. Can he broaden his coalition, without compromising the ideological purity that appeals to so many voters? The CWA is the latest organization to wager that the answer is yes.

Sanders’s campaign also announced an endorsement on Thursday from Democracy for America, a grassroots political organization founded by former Vermont governor Howard Dean.

The endorsements give a lift to Sanders’s campaign as the senator attempts to consolidate labor support and prove that he would do more to promote workers’ rights than Democratic frontrunner Hillary Clinton.

Sanders has worked hard to appeal to labor during his campaign. He has joined picket lines and styled himself as a defender of the middle class. Aides to Sanders announced on Wednesday night that the campaign has taken in more than 2 million contributions, reflecting a broad base of support.

For all that, however, Sanders lags far behind Clinton in amassing endorsements. He has only picked up backing from two members of Congress, compared to an avalanche of support for Clinton, as FiveThirtyEight points out. Clinton has so far won endorsements from 18 national labor unions and alliances. But on Thursday, it was Sanders who was touting his influential new backers, instead.