In 2016 the union’s super PAC spent $5 million backing Mr. Sanders in the primary contest against Hillary Clinton, a relative pittance in the world of super PACs (the one supporting Jeb Bush blew through $87 million). Still, it spent more money backing Mr. Sanders than was spent by any other super PAC on behalf of Mrs. Clinton or other Democrats in the run-up to the Iowa caucuses.

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The union’s super PAC “will be activated” on behalf of Mr. Sanders, said Bonnie Castillo, the union’s executive director. But she said union members and the super PAC would not attack Mr. Sanders’s rivals for the Democratic presidential nomination.

“We’re not going negative,” she said. “We are a very positive force. It’s a reflection of who we are as a profession. We are healers.”

When talking to voters, Mr. Sanders often rails against the influence of money in politics, vowing to overturn the Supreme Court’s Citizens United ruling in 2010 that opened the super PAC era in American politics. Since Mr. Biden dropped his resistance last month to receiving the support of such groups, Mr. Sanders has become more pointed.

“I don’t need a super PAC,” he said during a town hall-style event last month in Marshalltown, Iowa, in response to a question from the audience about Mr. Biden. “I am not going to be controlled by a handful of wealthy people. I will be controlled by the working people of this country.”

This weekend, he continued the theme, saying, “We don’t have a super PAC,” at a rally in Coralville, Iowa.