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.