Despite union opposition, the Daughters of Charity Health System’s board of directors approved on Friday the sale of its six California hospitals – including St. Vincent Medical Center near downtown Los Angeles – to Prime Healthcare Services of Ontario.

Other hospitals to be sold include St. Francis Medical Center in Lynwood and four Bay Area facilities. The Service Employees International Union immediately said they will fight the proposed sale, which they say is not in the public interest.

The healthcare workers and other opponents say Prime Healthcare, a for-profit company that operates 29 hospitals in California and other states, has a record of buying up distressed facilities and then cutting patient services, hiking up prices and laying off workers. The union said it will air television ads in Sacramento and San Francisco urging California Attorney General Kamala Harris to reject the sale.

Even before the sale was announced, the union lobbied against a sale to Prime, citing an investigation by the federal government against Prime into allegations of overbilling Medicare by improperly diagnosing patients.

“It’s disappointing and hard to understand how Daughters of Charity’s current owners could turn their back on 100 years of serving the poor by selling to a company with Prime’s history,” said Dave Regan, president of SEIU-United Healthcare Workers West.

Prime Healthcare said in a statement that it will maintain all existing services for at least five years and work to “substantially protect” jobs, while maintaining all collective bargaining agreements and contracts with workers and assuming all pension liabilities for current and retired workers. The hospital chain says that SEIU has been waging a “corporate campaign” against Prime Healthcare, and that the improper billing allegations are “baseless.”

Prime does have the support of some healthcare workers. The California Nurses Association and National Nurses United reached an agreement with Prime that the nurses say includes a commitment to preserve hospital services and labor standards.

“Prime Healthcare represents the best option to preserve vital healthcare services in our community,” said Maria Canonizado, a resident nurse at O’Connor Hospital in San Jose. “Prime has committed in writing to operate these vital hospitals in our communities.”

The DCHS sale will add to Prime Healthcare’s 2.5 billion health system that owns and operates 29 hospitals in nine states. The sale must also be reviewed and approved by the California Attorney General.