Emblazoned with the phrase "the most trusted profession trusts Bernie," the buses are a part of a nearly $2-million effort to elect Sanders by a labor union that represents registered nurses, about half of whom are from California.
The super PAC funded by National Nurses United is one of several groups working independently of the Sanders campaign to elect the Vermont senator. Though super PACs support every presidential candidate, including Sanders’ Democratic rival,
At virtually every campaign stop, Sanders decries the consequences of Citizens United vs.
At a rally in Las Vegas last weekend ahead of Saturday's Democratic caucuses, Sanders presented himself as the only candidate who hasn't turned "to the moneyed class" to help finance his run.
"We don't need a super PAC; we don't want a super PAC," Sanders told a roaring crowd. "We have chosen to go in a different direction."
In the risers behind him sat more than a dozen nurses in red scrubs that featured the hashtag #FeelTheBern. After the speech, they boarded the big red bus parked outside and spent the day talking up Sanders to voters.
Clinton's campaign and her supporters have slammed the nurses union, calling its spending on Sanders hypocritical.
"It's disappointing that Bernie Sanders continues to claim he doesn't have super PAC support when a super PAC has already spent more than $1.5 million boosting his campaign," said Clinton spokesman Josh Schwerin.
But Sanders and his backers insist that there is a key difference between the assistance he is receiving from such groups and the super PACs supporting his rival.
While Priorities USA Action, a pro-Clinton group run by a top aide to her 2008 campaign, has collected more than $40 million from wealthy donors including George Soros,
"We're the gnat compared to the sledgehammer," said Deborah Burger, the group's co-president, who said members voted to endorse Sanders because he has pledged to institute health insurance for all, paid for by the government.
Her group was formed in 2009 by a merger of three smaller unions, including the California Nurses Assn., which has funded campaigns opposing Republican gubernatorial candidate Meg Whitman and initiatives of former Gov. Arnold Schwarzenegger.
When asked, Sanders has said that he has not raised money for the nurses union, and pointed out that none of his close associates are running it.
His argument holds up, said Viveca Novak, editorial and communications director at the Center for Responsive Politics, which tracks campaign money.
"Priorities USA is filled with people who have been close to the Clinton campaign," Novak said. "The NNU was formed before Sanders ran for president. They have previously spent money on other candidates and campaigns. And it's not an entity that is staffed by people who are close to him."
But, she added, Sanders has also likely benefited from spending against Clinton by groups that represent exactly the interests he rails against. Republican-funded groups have spent more than $4 million in advertisements attacking Clinton, a point Clinton brings up often on the campaign trail.
Earlier this year, Sanders angered some liberals when he called Planned Parenthood and the Human Rights Campaign, both working to elect Clinton, "establishment" organizations.
He didn't acknowledge the many groups working on his behalf, including the Progressive Democrats of America, the Communication Workers of America and the American Postal Workers Union.
One organization that endorsed Sanders, the progressive group MoveOn.org, has asked its members to lobby Democratic superdelegates to support whichever candidate wins the most votes in the primaries when they help select the party's presidential nominee.
Party rules allow such delegates, who represent a third of the total delegates needed to win the nomination, to back whoever they wish, no matter who wins the popular vote in their states' nominating contests.
The group said it decided not to form a super PAC to support Sanders precisely because of his views on outside spending. That decision greatly limits how much money the organization can spend.
"But that's what the Sanders campaign wants," said Ben Wikler, the group's Washington director. "They want people to get directly involved."
Luci Riley, 48, a nurse from Richmond, Calif., said she and other NNU members have made small contributions to fund pro-Sanders advertisements but have also "put in the hours and the manpower."
"I'm getting Bernie blisters for this," said Riley, who was in Las Vegas over the weekend knocking on doors for Sanders.
Over breakfast one morning before fanning out across the city, she and other union members traded stories about how high insurance costs were making it difficult for them to care for patients.
They shook their heads as Burger shared a story about a patient who couldn't afford cancer medicine that had already been prepared for him. The patient was sent home, Burger said, and the medicine was thrown out.
She said she believes that part of her duty as a nurse is "advocacy beyond the bedside." That means getting involved in political campaigns that can help change the lives of her patients, she said.
"Even those who have insurance are underinsured," said Mary Roth, another nurse from Richmond. "If Bernie is elected, people won't be coming in as sick. He'll change things for us."
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