What organizers are calling the largest-ever strike by U.S. mental health care workers began Monday in California, when 2,600 employees of the hospital chain Kaiser Permanente walked off the job. The strikers, all affiliated with the National Union of Healthcare Workers (NUHW), accuse Kaiser Permanente of failing to keep its mental health services adequately staffed.

Many regions of the country are running low on mental health care workers, but NUHW says Kaiser Permanente's failure to recruit more staff for its mental health services is directly endangering patients.

NUHW employees have been attempting to negotiate a contract with Kaiser for more than four years. And this is not their first strike against the company. But it will be the first NUHW work stoppage to last a whole week — a clear escalation in the ongoing labor dispute.

In addition to the 2,600 mental health clinicians on strike, NUHW said that 700 other members would refuse to go to work this week. They will return to work the following Monday. In the meantime, California media have reported that Kaiser locations will remain open to provide emergency services, while non-urgent appointments will be rescheduled.

Mental health professionals are in short supply nationwide. In 2013, 91 million U.S. adults lived in regions of the country experiencing a shortage of mental health care providers, according to the Department of Health and Human Services. Philip Muskin, a professor of psychiatry at Columbia University's medical school, said the shortage is likely to "only get worse."

"There aren't enough psychiatrists," he said. "That's not true everywhere — there are enough psychiatrists in Manhattan — but there are many areas that are terrible underserved."

NUHW President Sal Rosselli said the point of the strike is “to focus attention on critical patient care for mental health care patients” — particularly on the problem of inadequate staffing, which the union says presents a direct threat to the mental health of Kaiser patients.

“We’ve been bargaining with Kaiser for over four years now,” he said. “They’ve dismissed these concerns, and the issue is getting far worse."

A November 2014 report available on the union’s website describes the lack of staff as a “crisis,” and accuses the hospital chain of violating both state and federal law by failing to hire more mental health care professionals. Kaiser has previously been penalized by state regulators for inadequate staffing. In September 2014 the company agreed to pay $4 million in fines to California’s Department of Managed Health Care (DMHC).

Kaiser now says it has increased the number of therapists on staff by 25 percent between 2011 and 2014, although NUHW argues the total mental health workforce — including therapists, social workers, marriage therapists and others — has increased by just 11.8 percent. Rosselli told Al Jazeera that such an increase just barely keeps up with the client growth brought on by the Affordable Care Act and hasn’t ameliorated the problem. He expects DMCH will be issuing another report on staffing issues at Kaiser in the near future.

In response to a media inquiry regarding NUHW’s claims, a spokeswoman from Kaiser Permanente provided a statement accusing the union of being "unwilling or unable to reach a fair agreement concerning a contract covering our employees."

"Not only are NUHW’s allegations factually wrong, but their tactics show that NUHW is not committed to working together to improve the quality, access, and affordability of mental health care,” the company statement said. "Although NUHW has been using intimidation and obstructionism to try to achieve its goals, we will not let that stop us from continuing to make progress on addressing the national challenge of providing high-quality mental health care."

Clement Papazian, an NUHW member who practices emergency psychiatric care for Kaiser Permanente, said the strike was primarily focused on improving conditions for patients.

“Ultimately the providers that do this work don’t get into it if they’re not passionate about it,” he said. “These are highly educated folks who could be doing a lot of other things to make money, but they’ve chosen to go into this line of work. They have a mission to provide these services."