Boston ? Among the many hurdles facing President Barack Obama’s plan to revamp the nation’s health care system is a shortage of primary care physicians — those legions of overworked doctors who provide the front line of medical care for both the sick and those hoping to stay healthy.

As Massachusetts’ experience shows, extending health care to 50 million uninsured Americans will only further stress the system and could force many of those newly insured back into costly emergency rooms for routine care if they can’t find a primary care doctor, health care observers said.

Massachusetts, home of the nation’s most ambitious health care law, has seen the need for primary care doctors shoot up with the addition of 428,000 people to the ranks of the insured under a 2006 law that mandates health care for nearly all residents.

To keep up with the demand for primary care doctors, the country will need to add another 40,000 to the existing 100,000 doctors over the next decade or face a soaring backlog, according to Dr. Ted Epperly, president of the Kansas-based American Academy of Family Physicians.