WASHINGTON -- During the debate over the 2010 federal health care overhaul, Democrats promised that illegal immigrants wouldn’t be among the 27 million people who’d gain coverage. President Barack Obama repeated that pledge last month when he outlined his immigration plan.

But while federal law generally bars illegal immigrants from being covered by Medicaid, a little-known part of the state-federal health insurance program for the poor pays about $2 billion a year for emergency treatment for a group of patients who, according to hospitals, mostly comprise illegal immigrants. Most of it goes to reimburse hospitals for delivering babies for women who show up in their emergency rooms, according to interviews with hospital officials and studies.

The funding – which has been around since the late 1980s and is less than 1 percent of the cost of Medicaid – underscores the political and practical challenges of refusing to cover an entire class of people. Congress approved the program after lawmakers required hospitals to screen and stabilize all emergency patients regardless of their insurance or citizenship status.

Some groups say the services encourage people to cross the border for care, while advocates for immigrants say the funding is inadequate because it doesn’t pay for prenatal care and other vital services.

“We can’t turn them away,” said Joanne Aquilina, the chief financial officer of Bethesda Healthcare System in Boynton Beach, Fla., which sees many illegal immigrants because of its proximity to farms where they harvest sugarcane and other seasonal crops.

Nearly one-third of Bethesda Hospital East’s 2,900 births each year are paid for by Emergency Medicaid, the category that covers mainly illegal immigrants. The category includes a small proportion of homeless people and legal immigrants who’ve been in the country less than five years.

Hospitals can’t ask patients whether they’re illegal immigrants, but instead determine that after checking whether they have Social Security numbers, birth certificates or other documents.

“We gather information to qualify patients for something and through that process, if you really hit a dead end, you know they are illegal,” said Steve Short, the chief financial officer at Tampa General Hospital.

A 2007 medical article in the Journal of the American Medical Association reported that 99 percent of those who used Emergency Medicaid during a four-year period in North Carolina were thought to be illegal immigrants.

The Federation for American Immigration Reform, which seeks to limit immigration, said the funding led more women to give birth in the United States, especially since they knew that children born here would be American citizens. The group believes that tens of thousands of “anchor babies” are born each year to illegal immigrants who hope that giving birth to children recognized as citizens will help the women gain legal status themselves.

Anyone born in the United States is a U.S. citizen. It’s unclear how many mothers later get green cards or become citizens.

The Federation for American Immigration Reform doesn’t dispute hospitals’ right to be reimbursed for care they’re required to provide.

“Our focus should be that you could save this money if you prevent the illegal immigration from happening in the first place. You can’t do it after the fact,” said Jack Martin, the special projects director for the organization.