(CNN) Democrats running for president have said they would support extending government health care coverage to undocumented immigrants -- a big shift, since undocumented immigrants currently have little access to federal programs.

They don't qualify for Medicare, aren't eligible for federal Affordable Care Act subsidies and generally can't enroll in Medicaid, which provides coverage for low-income people. Even many green card holders typically have to wait five years before they can enroll in Medicaid -- and President Donald Trump has changed immigration policy that could discourage even eligible immigrants from signing up.

What that means, today, is that many undocumented immigrants rely on emergency rooms -- which are required under federal law to screen and stabilize all patients who come through their doors -- when they get sick.

That ultimately costs the federal government billions in reimbursements to hospitals.

But Americans are divided about providing more comprehensive government-backedcoverage to the roughly 11 million undocumented immigrants in the US. Some 59% of Americans say that such coverage should not be available to undocumented immigrants, according to a CNN poll conducted in late June.

Among those who think it should be, there's a big disparity along party lines. Some 66% of Democrats supporting giving government coverage to the undocumented, but only 10% of Republicans feeling the same way.

The Democrats' position has given Trump fodder to attack his opponents.

"How about taking care of American Citizens first!?" he argued on Twitter after the first Democratic debate, in June.

Here's how undocumented immigrants access health care now:

Some undocumented immigrants have insurance

Some 45% of the non-elderly undocumented were uninsured in 2017, compared to 23% of lawfully present immigrants and 8% of citizens, according to the Kaiser Family Foundation.

Those who have coverage may obtain it at their jobs, through a family member's employer or from some state or local programs. They can purchase individual policies, but not on the Affordable Care Act exchanges, though this coverage tends to be very pricey, the Foundation found.

Some states and local governments provide coverage

Certain states, cities and counties do provide non-emergency services to undocumented residents, particularly low-income folks who would qualify for public benefits.

Six states, plus the District of Columbia, are using their own funds to expand their Children's Health Insurance Program, or CHIP, to all children who meet the income guidelines, regardless of immigration status.

California recently became the first state to extend Medicaid to undocumented low-income residents under age 26. Officials estimate the plan will cover about 90,000 young adults and cost the state $98 million in its initial year.

Other local governments, such as Los Angeles, San Francisco and Houston's Harris County, also run programs that provide care for the undocumented. New York City recently expanded its effort to connect uninsured residents with primary care doctors, regardless of immigration status.

Community clinics are another avenue -- and already get federal support

Undocumented immigrants can also turn to the nearly 1,400 community health centers around the nation, which typically provide primary and some specialty care and are often funded through federal grants and Medicaid.

"When you hear phrases like 'patchwork,' you're not kidding," said Nancy Berlinger, research scholar with The Hastings Center, a think tank focusing on social and ethical issues in health care, science and technology.

Undocumented immigrants spend much less on health care than US-born residents -- in part because they are generally younger and healthier and in part because they face many barriers to treatment, said Leah Zallman, a primary care physician with Cambridge Health Alliance, a safety net health care system in Massachusetts, who co-authored a review of studies on immigrant health care spending.

"They utilize shockingly little care overall," Zallman said.

The federal government also reimburses ERs for caring for the uninsured

ERs often seek federal reimbursement for treating the uninsured. One little-known program is called Emergency Medicaid, which spends about $2 billion annually to help pay for the treatment of about 100,000 people who would have otherwise qualified for Medicaid but for their immigration status, said Dave Chokshi, the chief population health officer at NYC Health & Hospitals and co-author of a report on the program published in the JAMA medical journal earlier this year.

"It makes more moral and clinical sense, but also more financial sense, to take care of people before they are in crisis," said Chokshi. "The care we provide right now is too often emergency care, rather than primary care."

It's a tiny share of the nearly $577 billion that the federal government and states spend on Medicaid. But most of the Emergency Medicaid fund is used to cover the care of the undocumented, with California receiving about half of the total, followed by New York, Texas and Florida, according to Chokshi.

The majority of the spending is on pregnancy-related treatment, but states can opt to broaden the scope of what they consider emergency services to chemotherapy or dialysis, for instance.

Still, some argue that policymakers should focus on citizens and legal residents.

"If you want to start covering people, you always want to start with those that are following the law and are supposed to be here," said Steven Camarota, director of research for the Center for Immigration Studies, which advocates for reducing immigration.