The president of a California union representing health care workers said members will picket outside Vice President Joe Biden Joe BidenBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Democratic groups using Bloomberg money to launch M in Spanish language ads in Florida Harris faces pivotal moment with Supreme Court battle MORE's upcoming fundraiser in Los Angeles set to be held at the home of a director of the Kaiser Foundation.

HuffPost reported Tuesday that the National Union of Healthcare Workers (NUHW), an Oakland-based union representing more than 3,000 Kaiser Permanente employees in the state, will hold an informal picket outside Biden's Wednesday event.

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The union has battled Kaiser for years over staffing levels in California clinics, according to HuffPost, and is reportedly pressuring Biden's campaign to side with union workers over his donors. The former vice president defended unions in recent days after criticism from President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE over several unions' endorsement of Biden's campaign.

“He should take our side in this dispute,” union president Sal Rosselli told HuffPost. “I think he should walk. He should cancel it.”

Biden's campaign did not immediately return a request for comment. The union has staged multiple protests in recent weeks at Kaiser's headquarters in California, accusing the company of unfairly cutting jobs in northern California.

Rosselli's union endorsed Biden's closest rival in the 2020 Democratic primary, Sen. Bernie Sanders Bernie SandersOutrage erupts over Breonna Taylor grand jury ruling Dimon: Wealth tax 'almost impossible to do' Grand jury charges no officers in Breonna Taylor death MORE (I-Vt.), during Sanders' 2016 run for president. Rosselli told HuffPost that Biden should refuse contributions from Kaiser executives until the union dispute is resolved.

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

Biden, who launched his campaign officially last month, currently sits at the top of most polls and has established a comfortable lead over Sanders, the only other contender in the crowded Democratic field regularly pulling double-digit support in surveys of early primary states.

He faced criticism from Sanders' campaign last month following his launch and subsequent fundraiser at the home of Comcast executive president David Cohen.