"We had hoped that workers would have a contract with wages and affordable health insurance before the debate next week," Susan Minato, co-President of Local 11, said in the statement. "Instead, workers will be picketing when the candidates come to campus."

Warren wrote on Twitter that Local 11 "is fighting for better wages and benefits—and I stand with them. 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."

Half an hour later, Sanders tweeted: "I stand with the workers of @UNITEHERE11 on campus at Loyola Marymount University fighting Sodexo for a better contract. I will not be crossing their picket line."

Yang also tweeted that he would not cross the Local 11 workers' picket line to attend the debate. "We must live our values and there is nothing more core to the Democratic Party than the fight for working people. I support @UNITEHERE11 in their fight for the compensation and benefits they deserve," he wrote.

"I won't be crossing a picket line," Biden tweeted. "We’ve got to stand together with @UNITEHERE11 for affordable health care and fair wages. A job is about more than just a paycheck. It's about dignity."

The planned demonstrations and candidates' ultimatums mark the second time a campus labor fight has upended plans for the December debate, slated to be the final party-sanctioned televised forum of the year.

After announcing the University of California, Los Angeles, as the debate's initial venue in late October, the Democratic National Committee backtracked two weeks later, deciding the university would not host the event.

AFSCME Local 3299, the University of California's largest employee union, had demanded a boycott of speaking engagements at the university after being locked in a dispute with the the 10-campus system for nearly three years.

"In response to concerns raised by the local organized labor community in Los Angeles, we have asked our media partners to seek an alternative site for the December debate," DNC senior adviser Mary Beth Cahill said in an emailed statement in November.

UCLA said in a statement it had "agreed to step aside as the site of the debate rather than become a potential distraction during this vitally important time in our country’s history."

Seven candidates have met the qualifying thresholds necessary to take part in the PBS NewsHour/POLITICO Debate — the smallest assembly of White House contenders set to appear on one debate stage thus far in the primary cycle.

Apart from Biden, Sanders, Warren and Yang, South Bend Mayor Pete Buttigieg, Minnesota Sen. Amy Klobuchar and billionaire environmental activist Tom Steyer have also qualified.