DES MOINES — As he swung through Iowa this week, Senator Bernie Sanders of Vermont rarely passed up a chance to bash the rising tide of money in politics, a system he said on Tuesday was “corrupt and undermining American democracy.”
At many of these stops, he was accompanied by members of National Nurses United, a seven-year-old union, fanning out from a bright-red bus in matching red scrubs to corral potential Sanders votes.
But the union is not just busing nurses into Iowa. The union’s “super PAC” has spent close to $1 million on ads and other support for Mr. Sanders, the Democratic presidential candidate who has inspired liberal voters with his calls to eradicate such outside groups. In fact, more super PAC money has been spent so far in express support of Mr. Sanders than for either of his Democratic rivals, including Hillary Clinton, according to Federal Election Commission records.
“I do appreciate the irony,” said RoseAnn DeMoro, the executive director of National Nurses United. “All things being equal, we would rather not be doing this. On the other hand, we want to see Bernie as president.”
Mr. Sander’s unlikely rise to super PAC pre-eminence is, in part, the story of an unusual alignment of strategies by different outside groups, including Republican ones eager to bloody Mrs. Clinton and lift Mr. Sanders, whom conservatives believe will be easier to defeat in a general election. While the nurses’ super PAC is the biggest left-leaning outside spender in the Democratic primary, conservative organizations have also spent at least $4.3 million attacking Mrs. Clinton in recent months.
One recent online ad from the Republican super PAC American Crossroads has assailed Mrs. Clinton for her Wall Street speaking fees — echoing an argument Mr. Sanders often makes against her. Another conservative group, Ending Spending, bankrolled by the Wyoming billionaire Joe Ricketts, has begun a $600,000 campaign in Iowa highlighting Mr. Sanders’s promises to raise taxes on the rich and provide free public college tuition, calling him “too liberal for Iowa.” But the ad’s language and imagery, including a contented-looking superrich couple hugging in front of a mansion and expensive cars, has led some Democrats to believe it is actually meant to bolster Mr. Sanders.
But the super PAC spending by the nurses’ union also underscores an aspect of the Supreme Court’s Citizens United decision that Mr. Sanders rarely dwells on in his campaign speeches attacking the ruling. The same decision that gave corporations the ability to “buy and purchase the United States government” — as Mr. Sanders put it on a visit to Grinnell College on Monday — bestowed the same rights on labor unions, freeing them to spend unlimited money from their treasuries on election advertising.
While the vast majority of super PAC money still comes from wealthy individuals, union cash — pooled from the dues and contributions of members — has become a critical source of money for outside groups on the left. In 2012 and 2014, unions gave more than $200 million to super PACs. More than half of it went to union-controlled groups that spent tens of millions of dollars on advertising, mailers and other “independent expenditures.” So far in 2016, according to data collected by the Center for Responsive Politics, seven of the top 20 organizational contributors to super PACs were unions or their affiliates, not corporations.
“When you have hundreds of thousands or millions of dues-paying members, you can wield a significant amount of influence,” said Robert Maguire, an investigator at the center. “It’s the flip side of a lot of other spending we’re seeing this days.”
No union has spent as much money in the Democratic primary as National Nurses United, which was born out of a 2009 merger of three smaller unions and has unapologetically embraced liberal politics and movement-building. In 2011, union nurses provided health care at the Occupy Wall Street encampment in Lower Manhattan, and the organization has lobbied forcefully for single-payer health care and a financial transaction tax.
When most large labor organizations backed Mrs. Clinton, the nurses were among a handful to support Mr. Sanders, among them the Communications Workers of America and the postal workers’ union. They are guided, the nurses’ leaders say, by the principle that taking care of patients means taking care of the country, too.
“This sick government we have right now is killing our democracy,” said Jean Ross, a Minnesota nurse now serving as one of the group’s three co-presidents, at a Sanders event in Des Moines on Tuesday. The union’s bus stood outside a steelworkers union hall, where the red scrubs worn by the nurses speckled a room full of Carhartt jackets and denim. At any given time, as many as 30 nurses from around the country are on the campaign trail helping Mr. Sanders, rotating in on vacation days and weekends.
“We have a bus, he has a bus,” Ms. Ross said gaily. “Nurses get on, nurses get off.”
The group’s campaigning advocacy for Mr. Sanders has drawn charges of hypocrisy from Mrs. Clinton’s supporters. While Mr. Sanders frequently declares that he has no super PAC of his own, he has not publicly called on the nurses to refrain from their efforts on his behalf. He has welcomed their help, thanking the nurses by name in campaign speeches and referring to the union in one recent appearance as a “one of the sponsors” of his campaign.
“This is one of the prime examples of Senator Sanders saying one thing and doing another,” said Brian Fallon, a spokesman for Mrs. Clinton. “For months he had criticized super PACs and pledged to shun them in his campaign, but all along he has benefited from hundreds of thousands of dollars in independent expenditures from one of those very organizations.”
Mr. Sanders insists there is no contradiction.
“The difference is a pretty simple difference,” Mr. Sanders told reporters on Tuesday. “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.”
Mrs. Clinton has indeed helped raise money for a super PAC: Priorities USA Action, a group originally formed to help re-elect President Obama and now run by a former Clinton campaign aide. The group has raised more than $40 million since the beginning of last year, including seven-figure contributions from the kind of billionaire financiers Mr. Sanders delights in lampooning on the campaign trail. A research and rapid-response super PAC, Correct the Record, founded by the Clinton ally David Brock and funded by wealthy liberals, has also taken swipes at Mr. Sanders in recent days over his record on gun control.
Priorities USA has continued to husband most of its money for later in the campaign, however, in anticipation of a major general election battle with Mrs. Clinton as the nominee. Several other liberal groups have spent money on behalf of Mrs. Clinton, including Planned Parenthood’s political arm and the League of Conservation Voters. But those expenditures totaled only $847,000, according to spending reported to the elections commission through Monday, less than the amount spent by the nurses’ super PAC, National Nurses United for Patient Protection.
Ms. DeMoro, the nurses’ union official, scoffed at any comparison between the two efforts, one by nurses, the other by a political operative.
“This is almost a morality play,” Ms. DeMoro said. “It’s Clinton’s David Brock versus Bernie’s nurses.”
Correction: January 28, 2016
An earlier version of this article misspelled the name of an investigator with the Center for Responsive Politics. He is Robert Maguire, not McGuire.
Yamiche Alcindor and Sarah Cohen contributed reporting.