California is facing a doctor shortage. In some places, the population per physician is twice as large as what medical experts recommend. And while lesser trained nurse practitioners have filled similar gaps in other places, the Golden State has seen a protracted fight over how widely they should be used.

Leading the charge against them: doctors.

The California Future Health and Workforce Commission recommended in a February report that the state expand the role of nurse practitioners to combat a projected shortfall of more than 4,100 primary care clinicians over the next decade. The group says the state should provide them with "greater practice authority, with particular emphasis in rural and urban underserved communities," where access to a physician is especially lacking.

But the proposal – which is similar in nature to regulations in nearly half of U.S. states – is meeting resistance from doctors who question whether sufficient oversight can be put in place for such a dramatic change in the delivery of medical care.

Nurse practitioners in general are a bridge between registered nurses and physicians – a mid-level option that took hold in the 1970s and '80s to address a growing doctor shortage across the country. In order to become a nurse practitioner, one must go to nursing school to become a registered nurse then pursue a graduate degree – the equivalent of a master's degree – in nursing. The more advanced training prepares them to diagnose patients, order and interpret medical tests, provide treatment and write prescriptions, but each state sets its own limits on the scope of their practice.

In 22 states , the District of Columbia and in federal government facilities, nurse practitioners have autonomy to practice free from the supervision of physicians. California, generally a progressive state when it comes to policy, is among a dozen states that impose the strictest limitations on nurse practitioners.

States that impose strict limitations include Texas, Oklahoma and Missouri, all states that rank low in health care in the U.S. News Best States ranking . Health care in Texas ranks 37 and the state has an obesity rate of 33%, higher than the national average. Oklahoma ranks almost last, 47, in health care quality and has an obesity rate of 36.5%. Missouri health care ranks 41 and also has a higher-than-average obesity rate.

Unexpectedly among these states is California . Although its overall health care ranks seventh in the nation, access to care falls somewhat in the middle, a problem nurse practitioners say they can fix.

State law prohibits them from practicing medicine without the supervision of a medical doctor or doctor of osteopathic medicine, and while licensed nurse practitioners in Washington state, where health care ranks fourth and access to health care ranks 12th, have full authority to prescribe medications, those in California do not. Though California law mandates supervision, it does not specify what the oversight should look like, and many nurse practitioners and patients say they already have little interaction with supervising physicians.

Theresa Ullrich, a family nurse practitioner and immediate past president of the California Association for Nurse Practitioners, has been in the field for 20 years. She said it's surprising that California, a state so forward-thinking in many areas, is "so far behind the times" when it comes to the issue.

At the clinic where Ullrich works in Orange, California, her supervising doctor is in one and a half days a week and is gone for six weeks in the winter.

California Assemblyman Jim Wood, a dentist, introduced a bill during the last legislative session to update the regulations, saying he was "looking to try to get care to my constituents." The bill would remove the red tape mandating supervision and allow nurse practitioners to practice on their own.

Wood also noted that more providers will become needed as health coverage grows.

"As we expand coverage to more and more people, we're going to need more providers," Wood said. "We don't have the provider network to support it."

The shortage of doctors not only limits health care access but also limits the amount of nurse practitioners that can enter the field, since California law mandates that one doctor can oversee no more than four nurse practitioners. Ullrich said many nurses struggle to find doctors to work with, and even if they do, if the doctor leaves or retires, the nurse practitioner must find a new one or be forced to close the practice, leaving patients at risk.

Karen Bradley, the president of the California Association for Nurse Practitioners and a pediatric nurse practitioner herself, said there would be no major changes in responsibilities of nurse practitioners if the law mandating supervision were overturned. The bill would "put into law what we are already doing."

But the California Medical Association, a lobbying group representing doctors in the state, says it strongly opposes legislation that would grant nurse practitioners the full ability to practice independently.

The association says the bill "would remove critical patient protections" by allowing nurse practitioners to work without physician supervision and would not "achieve expanded access to care, maintain patient safety or promote affordability."

The bill allows nurse practitioners "to practice medicine with no statutory limitations on their scope of practice and without completing the necessary education and training that the Legislature has deemed as essential for physicians to safely practice medicine," the organization said in a statement.

Wood said lawmakers have been trying to compromise with the CMA so it can feel confident nurse practitioners can safely and successfully practice on their own.

The medical association wrote lawmakers in April expressing its concerns, stating that the Medical Board of California is charged with reviewing the quality of medical practice in the state, while the Board of Registered Nurses oversees the nursing practice but "lacks the experience and expertise to make determinations regarding the practice of medicine."

The association said physicians complete rigorous education and training and "must prove their competency throughout their program to progress." Doctors must undergo a minimum of three years residency before they are eligible for licensure, while nurse practitioners do not have a similar level of training and are not required to meet residency requirements before licensure.

Wood, however, insisted that the provisions in the bill would ensure that nurse practitioners were capable. Several amendments were added to address the medical association's concerns, including requiring that nurse practitioners pass a national certification exam and receive approval from a doctor that they are competent to practice, as well as implementing a mandatory three-year transition period under the supervision of a doctor before being able to practice fully independently.

The association continued to push back, urging the state to instead invest in programs that will bring doctors to underserved areas and provide funding for additional residencies.

The medical association did not respond to a request for comment.

Bradley and Wood agreed that the shortage needs to be addressed soon because it is costing the state millions of dollars. They said that people without regular access to primary care have nowhere else to turn but emergency departments and urgent care clinics. The misuse of the services leads to overcrowding in waiting rooms and increased health care costs.

According to the California Future Health and Workforce Commission, of which Wood is a member, if full practice authority for nurse practitioners is achieved by 2020, there would be 50,000 fewer emergency department visits, resulting in a savings of more than $58 million a year.