During Thursday night's Democratic debate, Sen. Bernie Sanders (I-Vt.) trotted out a variation on a line we've heard from him often on the campaign trail: "I'm the only candidate up here who doesn't have a super PAC."

As a loud critic of money in politics, Sanders loves to rip super PACs while on the stump. And he's right to suggest that super PACs backing rival Hillary Clinton's candidacy are raising piles of dough right now.

But Sanders' bid for the White House also has a super PAC supporting it. It's called National Nurses United for Patient Protection.

National Nurses United is a labor union of 185,000 registered nurses known for its progressive politics. It was the first national union to endorse Sanders' campaign for the presidency, though most big unions have lined up behind Clinton. NNU's members have become a fixture at Sanders' rallies in the early primary states, and they canvassed hard for him in Iowa, where he finished neck-and-neck with the presumed front-runner.

While NNU's money may not match the most powerful corporate super PACs, it's hardly insignificant cash. As Politico recently reported, the group pulled in $2.3 million in 2015, its largest single-year haul ever. It shelled out $1.3 million during the second half of last year, with $400,000 of it going to other groups that support Sanders.

The super PACs that have sprouted up post-Citizens United have let companies spend unlimited amounts of money on political activities. But the rules have allowed labor unions to do just the same. And though unions may be outgunned by corporate America when it comes to raising cash, candidates like Sanders can still benefit from organized labor's super PACs.

Clinton's campaign has pounced on Sanders' support from the nurses union, calling it a clear contradiction. In a statement sent out immediately after Sanders made his remarks during Thursday's debate, Clinton's campaign said the senator's claim that he doesn't have a super PAC "just isn't true."

Sanders, meanwhile, has said he doesn't see any hypocrisy at play.

“The difference is a pretty simple difference,” the Vermont independent recently told reporters. “Hillary Clinton goes out raising money for her own super PAC. I don’t have a super PAC, and in the best of all possible worlds, which I hope to bring about, we will get rid of super PACs. We will overturn Citizens United.”

In fairness to Sanders, there are differences between a super PAC like that of National Nurses United and one like Priorities USA, a group aligned with Clinton. Most of the money in the nurses super PAC likely comes from the dues that individual workers pay to their union, in small amounts each paycheck.

In contrast, 98 percent of the money raised by Priorities USA Action in the second half of 2015 came from donors giving $100,000 or more, as The Huffington Post recently reported. And 90 percent of its money came from donors forking over at least $1 million.

Read more updates on the Democratic debate below: