National Nurses United For Patient Protection, a independent expenditure group, or super PAC, has spent $569,000 backing Sanders. National Nurses United, the union associated with the group, endorsed Sanders in August
, the first national union to do so, and has since been seen supporting him at events across the country.
The backing offers proof that it will be difficult for Sanders to stick to a no-super PAC pledge.
One of the most well-received lines in Sanders' stump speech is when he blasts super PACs and suggests that they have a corrupting influence on the political system. Many of his supporters say one reason they like him is because of his view on super PACs, which are not subject to campaign finance limits.
"You are looking at the non-billionaire major candidate for president who does not have a super PAC," he told an audience in Boulder last month.
"I don't have a super PAC," he told reporters in Washington over the summer. "I am not going to have a super PAC."
Sanders campaign manager Jeff Weaver said the senator is "honored" to have the union's support. "They understand firsthand the need for health care reform in this country and which candidate as president would actually support single-payer health care for all and not attack it."
Weaver said that Sanders's campaign is not connected to the super PAC.
"Unlike others, we have not started a super PAC, are not coordinating with a super PAC, and we have not fundraised for a super PAC," Weaver said. "We stand by our position that we do not want the help of a super PAC."
Most of the group's spending has been on printing pro-Sanders literature and online and print advertising.
Michael Lighty, the nurses union political director, argued on Wednesday that the independent expenditure group -- which is often referred to as a super PAC -- was not one.
"This is not a super PAC. It is really a different animal," Lighty said. "This PAC predates the Sanders campaign. It was a PAC set up originally under the pre-Citizen United rules, so it is not a super PAC."
But according to the Federal Election Commission, that isn't the case. An FEC spokesman told CNN on Wednesday that National Nurses United For Patient Protection is, in fact, a super PAC, despite being established in 2009, before the 2010 U.S. Supreme Court decision that led to the proliferation of super PACs.
National Nurses United super PAC is different than those supporting other candidates, like Priorities USA, a PAC backing Hillary Clinton, and Right to Rise, a group backing Jeb Bush. Both of those organizations have raised millions of dollars with the blessing of their respective candidates and are generally run by former aides.
The nurses super PAC is funded by union members and does not coordinate with Sanders, nor does Sanders raise money for the group. The groups has also been active in past, spending $2.4 million in 2014 and $500,000 in 2012.
"Our organization exists to protect nurses and patients. No candidate has better amplified the critical issues nurses have voiced for many years than Bernie Sanders," Deborah Burger, co-president of National Nurses United, told CNN in a statement. "We are proud to support Bernie Sanders and will continue to do so."