The faceoff is schedule for Thursday in Los Angeles at Loyala Marymount University, where workers are planning a picket.
A Democratic presidential debate scheduled for next week in California was thrown into upheaval on Friday when all seven of the candidates who've qualified for the stage threatened not to attend because of a labor dispute at the site.
The Thursday debate is scheduled to be held at Loyola Marymount University in Los Angeles, where the food workers' union says it's been working without a contract.
The union, Unite Here Local 11, announced Friday that it would picket the debate, and Elizabeth Warren, Bernie Sanders, Joe Biden, Pete Buttigieg, Amy Klobuchar, Tom Steyer and Andrew Yang quickly said they would not cross that line.
Warren was the first to speak out, tweeting that the union "is fighting for better wages and benefits — and I stand with them." She also called out the Democratic National Committee, saying the "DNC should find a solution that lives up to our party's commitment to fight for working people. I will not cross the union's picket line even if it means missing the debate."
Warren's tweet was followed soon after by fellow progressive Sanders, who tweeted, "I stand with the workers" who are fighting a an LMU contractor, Sodexo, "for a better contract. I will not be crossing their picket line."
Yangalso tweeted in solidarity, and said he would not cross the picket line. "We must live our values and there is nothing more core to the Democratic Party than the fight for working people," Yang tweeted.
Candidate Julián Castro has not qualified for the debate, but urged his fellow candidates not to attend.
"No candidate for the Democratic nomination should cross a picket line," he tweeted.
In a press release, the union said its 150 cooks, dishwashers, cashiers and servers have been in negotiations with Sodexo, which runs the food service operation on campus, since March, but have yet to reach a deal. They began picketing on campus last month, and the release said Sodexo abruptly canceled scheduled contract negotiations last week.
"We had hoped that workers would have a contract with wages and affordable health insurance before the debate next week. Instead, workers will be picketing when the candidates come to campus," said Unite co-president, Susan Minato.
The original debate site was supposed to have been UCLA but it was move to LMU due to a labor negotiation dispute at UCLA.
The debate, which is being co-sponsored by Politico and PBS Newshour, was shaping up to have the smallest number of participants of the Democrats' 2020 campaign because of the higher qualification threshholds. Eight candidates hit the benchmarks to make the debate stage, and one of them, Sen. Kamala Harris, has dropped out of the race.
The seven remaining candidates who qualified by having 200,000 or more donors and hitting at least 4 percent in four national or state polls are Warren, Sanders, Yang, Joe Biden, Pete Buttigieg, Amy Klobuchar and Tom Steyer.
Last month's Democraticpresidential debate, the fifth of the cycle, featured ten candidates on stage.