WOOD LAKE, Neb.

There are just a handful of psychiatrists in all of western Nebraska, a vast expanse of farmland and cattle ranches. So when Murlene Osburn, a cattle rancher turned psychiatric nurse, finished her graduate degree, she thought starting a practice in this tiny village of tumbleweeds and farm equipment dealerships would be easy.

It wasn't. A state law required nurses like her to get a doctor to sign off before they performed the tasks for which they were nationally certified. But the only willing psychiatrist she could find was seven hours away by car and wanted to charge her $500 a month. Discouraged, she returned to work on her ranch.

"Do you see a psychiatrist around here? I don't!" said Osburn, who has lived in Wood Lake, population 63, for 11 years. "I am willing to practice here. They aren't. It just gets down to that."

But in March the rules changed: Nebraska became the 20th state to adopt a law that makes it possible for nurses in a variety of medical fields to practice without a doctor's oversight. Maryland's governor signed a similar bill into law this month, and eight more states are considering such legislation, said the American Association of Nurse Practitioners.

Now nurses in Nebraska with a master's degree or better, known as nurse practitioners, no longer have to get a signed agreement from a doctor to be able to do what their state license allows — order and interpret diagnostic tests, prescribe medications and administer treatments.

"I was like, 'Oh, my gosh, this is such a wonderful victory,' " said Osburn, who was delivering a calf when she got the news in a text message.

The laws giving nurse practitioners greater autonomy have been particularly important in rural states like Nebraska. About a third of Nebraska's 1.8 million people live in rural areas, and many go largely unserved as the nearest mental health professional is often hours away.

"The situation could be viewed as an emergency, especially in rural counties," said Jim P. Stimpson, director of the Center for Health Policy at the University of Nebraska, referring to the shortage.

Groups representing doctors, including the American Medical Association, are fighting the laws. They say nurses lack the knowledge and skills to diagnose complex illnesses by themselves. Dr. Robert M. Wah, the president of the AMA, said nurses practicing independently would "further compartmentalize and fragment health care," which he argued should be collaborative, with "the physician at the head of the team."

Dr. Richard Blatny, the president of the Nebraska Medical Association, said nurse practitioners have just 4 percent of the total clinical hours that doctors do when they start out. They are more likely than doctors, he said, to refer patients to specialists and to order diagnostic imaging like X-rays, a pattern that could increase costs.

Nurses say their aim is not to go it alone, but to have more freedom to perform the tasks that their licenses allow without getting permission from a doctor — a rule that they argue is more about competition than safety. 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. Three to 14 nurse practitioners can be educated for the same cost as one physician, said a 2011 report by the Institute of Medicine. In all, nurse practitioners are about a quarter of the primary care workforce, the institute said, 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 U.S. Supreme Court found that North Carolina's dental board did not have the authority to stop dental technicians from whitening teeth in nonclinical settings. 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."