Democratic presidential candidate Bernie Sanders (I-Vt.) renewed his call for the Employee Free Choice Act, which would “make it easier for workers to join unions.”

“There are millions of workers in this country, not everyone, but millions of workers who want to join a union but are unable to do so because suddenly and mysteriously a worker active in the organizing effort suddenly gets fired because she was late three years ago and we understand that employers take people into propaganda sessions,” Sanders said at a Oakland, Calif., rally held by National Nurses United, a union that endorsed Sanders.

“We understand that employers threaten workers. ‘You want a union? Good. You form a union, we’re shutting down here and we’re going to China.’ We understand that it is not uncommon that when workers in fact go through all of these hoops and form a union and negotiate a first contract, employers refuse to sit down and honestly negotiate that contract,” he added.

Sanders said the Employee Free Choice Act, originally introduced in 2009, is a simple bill.

“If 50 percent of the workers in a bargaining unit plus one sign a card saying they want a union, they will have that union,” he said. “You know what this right-wing attack on unions is about.”

Sanders claimed that GOP presidential candidate Gov. Scott Walker (R-Wis.) and others want to “destroy” unions because the trade unions are the “last line of defense for working people against corporate greed.”

Without a stronger trade union movement, Sanders said, the future of the middle class is in doubt. He argued that employers do not want to sit down and collectively negotiate given that a group is more powerful than an individual.

“If you go alone you’re nothing, but you can threaten to strike and you have leverage,” he said.

NNU Executive Director RoseAnn DeMoro said Sanders has a proven track record of “uncompromised activism” and advocacy for working Americans, which is a message that resonates with nurses.

“We assumed because we are a woman’s organization a lot of the nurses would find Hillary Clinton far more resonant because she would break the glass ceiling,” she said. “They were far more concerned about breaking Wall Street’s stranglehold on our economy than the glass ceiling.”

During the rally, Sanders addressed critics who doubt his ability to beat Democratic presidential candidate Hillary Clinton.