Nurses say their aim is not to go it alone, which is rarely feasible in the modern age of complex medical care, but to have more freedom to perform the tasks that their licenses allow without getting a permission slip from a doctor — a rule that they argue is more about competition than safety. They say advanced-practice nurses deliver primary care that is as good as that of doctors, and cite research that they say proves it.

What is more, nurses say, they are far less costly to employ and train than doctors and can help provide primary care for the millions of Americans who have become newly insured under the Affordable Care Act in an era of shrinking budgets and shortages of primary care doctors. Three to 14 nurse practitioners can be educated for the same cost as one physician, according to a 2011 report by the Institute of Medicine, a prestigious panel of scientists and other experts that is part of the National Academy of Sciences.

In all, nurse practitioners are about a quarter of the primary care work force, according to the institute, which called on states to lift barriers to their full practice.

There is evidence that the legal tide is turning. Not only are more states passing laws, but a February decision by the Supreme Court found that North Carolina’s dental board did not have the authority to stop dental technicians from whitening teeth in nonclinical settings like shopping malls. The ruling tilted the balance toward more independence for professionals with less training.

“The doctors are fighting a losing battle,” said Uwe E. Reinhardt, a health economist at Princeton University. “The nurses are like insurgents. They are occasionally beaten back, but they’ll win in the long run. They have economics and common sense on their side.”