Members of National Nurses United cheer as Democratic presidential candidate Bernie Sanders speaks during a campaign rally in Des Moines on Tuesday. (Joshua Lott /for The Washington Post)

People for Bernie is organizing volunteers through social media. National Nurses United is crisscrossing Iowa in its big red bus, working to turn out supporters at Monday’s presidential caucuses. Progressive Democrats of America is already focused on states later in the primary calendar, holding house parties in Alabama, Florida and Virginia.

As he barrels toward the Iowa caucuses, Sen. Bernie Sanders (Vt.) is being boosted by a wide constellation of independent groups aiming to compete with the organizational muscle arrayed behind former secretary of state Hillary Clinton, his Democratic competitor.

While Clinton has powerhouse allies such as Planned Parenthood, the National Education Association and two big-money super PACs on her side, the pro-Sanders effort is being driven by a combination of self-directed activists and liberal organizations such as MoveOn and Democracy for America.

The ad hoc network working on Sanders’s behalf is doing so in keeping with the spirit of his anti-establishment bid. But it is also employing professional political tactics, such as the use of entities that can raise and spend unlimited sums of money on campaigns as a result of the Supreme Court’s Citizens United decision.

In some ways, their efforts cut against Sanders’s insistence that, unlike Clinton, he does not have a super PAC flanking his campaign — a declaration he repeated Tuesday in Des Moines after leading a rally at a union hall attended by several nurses in their union’s signature red T-shirts. In other ways, they don’t. Although these entities can accept massive checks from individuals and corporations — a practice Sanders abhors — they do not appear to be doing so, relying instead on small donations from grass-roots supporters.

“The difference is a pretty simple difference,” he said. “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. I do not have a super PAC, I’ve never raised a nickel for a super PAC, I don’t want a super PAC.”

A super PAC financed by the nurses union already has plowed nearly $1 million into mailers, phone banking and a multistate bus tour to promote Sanders. Friends of the Earth Action, a nonprofit environmental group, spent nearly $22,000 to run a cable ad last week touting his record in Iowa and New Hampshire.

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Leaders of the nurses union and Friends of the Earth Action argue that they should not be lumped in with the single-candidate super PACs backing other White House hopefuls, which have been financed largely by six- and seven-figure donations.

“This is our dues money,” said RoseAnn DeMoro, executive director of National Nurses United (NNU). “Nurses aren’t millionaires. These are individual contributions that made this possible.”

Erich Pica, president of Friends of the Earth Action, said the nonprofit group paid for the ad out of small donations it receives from supporters. If the group does any further spending to support Sanders, it will voluntarily disclose the name of any contributor who gives more than $200 for that purpose, he said.

Still, Pica acknowledged that his group had “some hesitancy” about running an independent ad supporting Sanders because of the senator’s frequent condemnation of super PACs. (Friends of the Earth Action is a tax-exempt advocacy group, but like a super PAC, it can raise unlimited donations.)

“The other thing Senator Sanders has been saying on the campaign is that he can’t do it alone,” Pica said. “The work, we have to do together. And for us, Senator Sanders isn’t going to win this nomination on his own.”

In all, at least half a dozen independent groups — including some that are loosely affiliated — are mustering substantial campaigns on Sanders’s behalf, while hundreds of other hyperlocal endeavors, such as Brews for Bernie and Babes for Bernie, are doing their own organizing.

“We gave up trying to keep track of them all,” said Charles Lenchner, a Brooklyn-based political marketing consultant who co-founded People for Bernie, one of the earliest efforts to harness grass-roots support.

Lenchner started the project in April with activist Winnie Wong, a fellow Occupy Wall Street organizer, after they first tried to draft Sen. Elizabeth Warren (D-Mass.) into the race. When Sanders announced his campaign, People for Bernie helped supporters organize house parties and quickly established social-media profiles for a collection of constituency groups, including Millennials for Bernie and African Americans for Bernie.

The group was also the first to coin the hashtag #FeelTheBern, tweeting it in early May and then watching it go viral.

People for Bernie now uses its large online following to promote Sanders and help connect volunteers to the official campaign.

“We’re curating the news, supporting the grass roots, getting them information,” Wong said.

The project has received a boost from the nurses union, which is hosting Wong on its Iowa bus tour this week and has allocated $45,000 to People for Bernie to run online ads to expand its social media presence, said Michael Lighty, NNU’s policy director.

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Because People for Bernie is not set up as a legal entity or registered as a political committee, NNU gave the money to Progressive Kick, a liberal super PAC, to administer the effort, Lighty said. Joshua Grossman, president of Progressive Kick, confirmed the arrangement but declined to comment further.

In many ways, the 185,000-member nurses union is functioning as the hub of the pro-Sanders efforts. At an event in Cedar Falls, Iowa, on Sunday, Sanders pointed out several supporters in the audience wearing NNU’s red T-shirts and hailed the union as “one of the sponsors of my campaign.”

DeMoro, NNU’s executive director, said that the union will spend “whatever we need” to help elect Sanders, and that members are buoyed by his pledge of “Medicare for all.”

“We have had high-pitched campaigns, but I have never seen the grass-roots enthusiasm like this,” she said. “Bernie is golden with our members.”

The union has been targeting voters in Nevada and Iowa with mailers and phone calls, and it plans to be active in states including South Carolina, Colorado and Minnesota.

“We really want to be in every single primary or caucus state,” Lighty said.

In Illinois, NNU is funding a group called Reclaim Chicago to organize community leaders, Lighty said, one of several organizations it is backing to do specific pro-Sanders campaigns. He declined to name them all, citing internal strategy.

Lighty is also on the advisory board of Progressive Democrats of America, a political action committee that launched an effort to draft Sanders to run in the spring of 2014.

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The PAC is now active in 31 states in support of Sanders, with the aim of organizing volunteers in places holding primaries and caucuses later this spring, where the Sanders campaign has less of a presence.

“We try to keep our eyes peeled for where we see there may be not a lot of activity and where we want to fire people up,” said Donna Smith, PDA’s executive director.

That has not been a hard lift, she added.

“We did not even see this level of excitement going into 2008 with Obama’s campaign,” Smith said. “Sanders’s message is resonating in a very deep way with people, much more than many of us may have anticipated.”

One immediate byproduct: Sanders’s campaign has helped lift the profile of Democratic Socialists of America, a nonprofit advocacy group that has gained hundreds of members since he began running.

The organization has spent about $42,000 in support of Sanders, largely on volunteer-staffed training sessions to educate people about socialism, or tables at farmers markets and other community events to promote his candidacy, said Maria Svart, the group’s national director.

“We believe in spreading socialist values, because we believe many Americans, if they understood socialism, would like it,” she said. “That’s what so exciting about Bernie. Our perspective is, whether he’s elected or not, we need to have an organized, aware grass-roots movement.”

John Wagner in Des Moines and Anu Narayanswamy in Washington contributed to this report.