NUHW is locked in a bitter, yearslong fight with Kaiser over mental health staffing levels in the health care provider’s California facilities. Therapists waged a statewide five-day strike in December, and a one-day work stoppage in Pasadena last month.

The National Union of Healthcare Workers plans to hold an informational picket line outside a Biden fundraiser slated for Wednesday in Los Angeles, according to Sal Rosselli, the union’s president. The fundraiser is to be held at the home of a director of the Kaiser Foundation , which operates hospitals as a subsidiary of Kaiser Permanente.

Former Vice President Joe Biden may consider himself a “ union man ,” but one California union won’t be giving him a warm welcome this week.

ASSOCIATED PRESS Democratic 2020 presidential candidate Joe Biden won't get a warm welcome from a California union this week. The National Union of Healthcare Workers plans to picket a Biden fundraiser.

Rosselli told HuffPost his union first reached out to Biden last week about the fundraiser to be hosted by Dr. Cynthia Telles, a UCLA School of Medicine faculty member who sits on Kaiser’s board. According to an invitation, contribution levels for the event range from $250 per person to $2,800, which is the maximum contribution for the presidential primary allowed under election law.

Rosselli said the union has not heard back from the campaign.

“He should take our side in this dispute,” Rosselli said. “I think he should walk. He should cancel it.”

Neither the Biden camp nor Telles immediately responded to requests for comment.

The NUHW represents 3,500 psychologists and other workers at Kaiser facilities. The union has long alleged that Kaiser understaffs its mental health clinics, leading to long waits for vulnerable patients. The union has demanded that Kaiser hire more therapists and put pay and benefits on par with other employees in medical care.

“It doesn’t matter what [a patient’s] situation is,” one therapist told USC Annenberg Media in April. “They could be struggling with parenting, they could be going through bereavement, they could have a bipolar disorder. They’re still gonna have to wait three months.”

In a statement, Kaiser Permanente spokesperson John Nelson called the picket a “publicity stunt by union leadership disguised as a bargaining tactic.” He added that Kaiser was making progress toward a new contract with the union. “What’s so disappointing is that despite that alignment, union leadership has decided to engage in this unproductive behavior,” Nelson said.

The tiff over the California fundraiser is another sign of the trouble Biden may run into with the progressive left as he works his donor network.

The former veep kicked off his campaign with a show of union support last week, winning the endorsement of the International Association of Fire Fighters and speaking at a Teamsters hall in Pennsylvania. But as HuffPost reported, the early Democratic front-runner is bound to have a harder time rounding up the support of progressive unions uneasy with his centrist track record and corporate ties.

Some of his top opponents are already using that line of attack. Sen. Elizabeth Warren (D-Mass.) criticized Biden last week, describing him as a friend to credit card companies. Meanwhile, the campaign of Sen. Bernie Sanders (I-Vt.) took a shot at Biden for holding a fundraiser at the home of a Comcast executive.

Rosselli, whose union endorsed Sanders in the 2016 election, argued that Biden shouldn’t take money from Kaiser executives or board members until the health care provider resolves the dispute with the union amicably.

“I think he should step up and get a commitment,” Rosselli said.

This story has been updated with comment from Kaiser Permanente.