The presidential campaign of Sen. Bernie Sanders is distancing itself from a liberal super PAC with plans to support his bid, saying it doesn’t want the group to spend money on its behalf.

Sanders has made opposition to big money in politics a cornerstone of his White House run, and often boasts that he is the only Democratic presidential candidate without a super PAC in his corner.

Close video Rapper 'Killer Mike' endorses Bernie Sanders Ebony’s Kierna Mayo explains what rapper and activist “Killer Mike’s” endorsement means for the Black Lives Matter movement. Ebony’s Kierna Mayo explains what rapper and activist “Killer Mike’s” endorsement means for the Black Lives Matter movement. share tweet email Embed

Already, though, he’s faced questions over hundreds of thousands of dollars spent on his behalf by a super PAC affiliated with National Nurses United, a nurses union that has endorsed him. Sanders seemed to make an exception to his no-super PAC rule for them, arguing a group funded by union dues from nurses is not anyone’s idea of big corporate money.

“They are nurses and they are fighting for the health care of their people. They are doing what they think is appropriate. I do not have a super PAC,” he told CNN.

Now, Sanders’ campaign is drawing a harder line with the California-based super PAC Progressive Kick, which endorsed Sanders in April and told the AP this week that it plans to spend money supporting his candidacy, despite the candidate’s opposition to super PACs and his push to get big money out of politics.

In a statement to MSNBC Friday, Sanders campaign manager Jeff Weaver said thanks but no thanks to the group.

“As he says in every speech he gives, Sen. Sanders doesn’t want any help from super PACs, including Progressive Kick,” Weaver said. “This campaign is driven by millions of people around the country who want to see an end to the establishment politics that created super PACs in the first place. Unlike our Democratic opponents, our campaign has not started a super PAC, is not fundraising for a super PAC, and is not actively coordinating with a super PAC.”

Progressive Kick expected Sanders’ opposition and will move ahead with its plans anyway. But its president, Joshua Grossman, told MSNBC his group is different from typical super PACs, explaining he has no current plans to use donations from wealthy individuals to fund the PAC’s work for Sanders. He also said that unlike other candidates’ super PACs, which are typically run by former staffers and have the implicit blessing of the candidate, he has never met Sanders and no one from the Sanders campaign asked him to do this.

The PAC spent $2.2 million supporting progressive Democrats in 2014, according to the Center for Responsive Politics, with a majority of its donations coming from National Nurses United For Patent Protection, the union super PAC.

Federal law generally prohibits candidates from directing super PACs’ activities, so the Sanders campaign can do little more than ask the group to stop.

Close video Sanders tries to take control of immigration conversation Arturo Carmona of the Bernie Sanders campaign breaks down the Democratic presidential candidate’s immigration plans and whether they realistic. He says they are “comprehensive and humane.” Arturo Carmona of the Bernie Sanders campaign breaks down the Democratic presidential candidate’s immigration plans and whether they realistic. He says they are “comprehensive and humane.” share tweet email Embed

There is precedence, however, for independent expenditure groups to heed those calls. In 2008, a soft money group (super PACs did not exist at the time) set up to support to Barack Obama’s presidential campaign dramatically scaled back their plans after the then-senator publicly objected.

And earlier this year, a lawyer working for Sanders’ campaign sent a cease and desist letter to a separate super PAC supporting his candidacy that campaign finance watchdogs described as “shady.”

Democratic front-runner Hillary Clinton has two deep-pocketed super PACs closely tied to her campaign. One exploits a loophole in campaign laws to coordinate directly with her campaign’s staff. Clinton donors are encouraged to contribute to the other.

Former Maryland Gov. Martin O’Malley, who is currently in third place in the polls, also has a super PAC started by former staffers that is dedicated to supporting his presidential bid.