"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."

The Democratic National Committee said in a statement that it was working with all parties involved to find "an acceptable resolution" that will allow the debate to go forward.

"The DNC and LMU learned of this issue earlier today, and it is our understanding this matter arose within the last day," Xochitl Hinojosa, the DNC communications director, said in the statement. "While LMU is not a party to the negotiations between Sodexo and Unite Here Local 11, Tom Perez would absolutely not cross a picket line and would never expect our candidates to either."

Massachusetts Sen. Elizabeth Warren tweeted 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, Vermont Sen. Bernie 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."

Tech entrepreneur Andrew 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," former Vice President Joe 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."

Billionaire envrionmental activist Tom Steyer tweeted that if the dispute between Local 11 and Sodexo "is not resolved before the debate, I will not cross the picket line. I trust the DNC will find a solution ahead of the debate, and I stand with @LoyolaMarymount workers in their fight for fair wages and benefits."

"I take the debate stage to stand up for workers’ rights, not to undermine them," tweeted South Bend Mayor Pete Buttigieg. "I stand in solidarity with the workers of @UNITEHERE11 at Loyola Marymount University and I will not cross their picket line."

Speaking at a roundtable of labor leaders in Miami, Minnesota Sen. Amy Klobuchar said: "I don't believe we should cross a picket line. So I would encourage the DNC to try to work this out to find a new location, or they're going to have to figure out how to resolve this."

The California Labor Federation, which is made up of 1,200 affiliated unions, had urged the White House contenders to not participate in the debate amid the protests, tweeting: "Every democratic candidate has vowed to fight for working people. It's time to put those words into action."

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 DNC 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."

The DNC also officially announced on Friday that the seven candidates met the qualifying thresholds necessary to take part in the PBS NewsHour/POLITICO Debate — the smallest assembly of competitors set to appear on one debate stage thus far in the primary cycle.

Alex Thompson contributed to this report.