To be sure, it's estimated that the Affordable Care Act would greatly increase the number of insured Americans. The law has a provision that requires most Americans to be insured or face a tax penalty. It also calls for an expansion of Medicaid, a government-funded program that covers the health care costs of low-income and disabled Americans. Additionally, starting in 2014, there will be tax credits to help middle-class Americans buy coverage.
The Supreme Court is expected to hand down a decision this month on whether to uphold the law completely or strike down parts or all of it. If it survives, about 93 percent of all non-elderly, legal U.S. residents will be covered by 2016. That's up from 82 percent this year.
Still, millions of illegal immigrants won't qualify for coverage. This population will account for roughly 26 percent of those who will remain uninsured, according to Urban Institute, a nonpartisan think tank.
And many legal U.S. residents will go without insurance, too. About 36 percent of the population that remains uninsured will qualify for Medicaid but won't sign up for various reasons. Others likely will make too much money to qualify for assistance but be unable to afford coverage.
Here's a look at some of the groups that will likely remain uninsured if the law survives:
ILLEGAL IMMIGRANTS • More than 11 million unauthorized immigrants live in the United States, according to the Pew Hispanic Center, a nonpartisan research center. That amounts to nearly 4 percent of the total population. But there are no provisions that address illegal immigrants in the health care law.
They won't be able to sign up for Medicaid. They won't be eligible for the tax credits to help buy coverage. And they won't be able to use online marketplaces that the government will set up in order for people to get coverage in a process that's similar to buying plane tickets on travel web sites. Those online exchanges, much like the tax credits, will require proof of citizenship.
"They will still need to find alternative ways to seek care because nothing in the law really expands coverage and affordable coverage options for undocumented immigrants," says Sonal Ambegaokar, a health policy attorney with the National Immigration Law Center in Los Angeles.
The topic is a politically divisive issue. On one side, there are people who say that the government should provide health care for all U.S. residents legal or not. The other side contends that doing so could take valuable resources away from U.S. citizens.
"Because of the limited supply of health care, we're almost in a sociological triage," says Bob Dane, spokesman for the Federation for American Immigration Reform, a national group that calls for stricter immigration laws. "It begs the question, 'Who do we serve, who do we serve first and who is not entitled?'"
Researchers have found that immigrants tend to use the health care system less than legal residents. Illegal immigrants, in particular, tend to avoid using the health care system until they have to, favoring home remedies first or making cash payments to providers when they need care. That population also is younger, so it generally has fewer health care needs, says Timothy Waidmann, a researcher with Urban Institute.
The think tank, using federal government survey data, estimates that illegal immigrants accounted for an estimated $18 billion of the $1.4 trillion spent on health care in the United States in 2007. That adds up to less than 2 percent of total spending.
Some say excluding illegal immigrants from the overhaul will keep some legal residents uninsured, too. Ambegaokar, the Los Angeles attorney, points to parents who are illegal immigrants but have children who are legal citizens because they were born in the United States.
If the parents are not eligible, they may not know that their kids qualify. And in other instances, if one child is legal and the other is not, the parents may decide not to sign up either to avoid playing favorites.
"The goal is to enroll everybody who is eligible," Ambegaokar says. "But when you make systems complicated and require proof of ID, you're going to inevitably keep out people who should be in."
LOST IN TRANSLATION • Medicaid, which currently covers more than 60 million people, is expected to add about 17 million more people to its program by 2016 if the law is upheld, according to the nonpartisan Congressional Budget Office, which researches budgetary issues for Congress.
But people are still expected to fall through the cracks. That's because the requirements and process for signing up for Medicaid can be confusing. And while the overhaul aims to make the process easier, it won't smooth out all the wrinkles.
The problem? Many people don't realize that they qualify for coverage. And that likely will still be the case, albeit to a lesser extent, after Medicaid expands.
Coverage depends on how someone's income stacks up to federal poverty guidelines, which can be obscure to the average person. Plus, because income can fluctuate, someone could qualify one year but not the next.
"Regardless of how much outreach you do ... you're never going to get perfect enrollment," Matthew Buettgens, another Urban Institute researcher, says.
Staying enrolled can be another hurdle. Medicaid recipients have to re-enroll, sometimes more than once a year. They can be dropped if they miss deadlines, submit incomplete forms or if paperwork doesn't catch up with them after they move something poor families tend to do more frequently than the average American household.
Leeanna Herman learned this when an unexpected $300 doctor bill arrived in the mail. The Bakersfield, Calif., resident was pregnant and unemployed and didn't know her government-funded health coverage had lapsed.
"I was freaking out," says Herman, 23, who went a month without coverage because she missed the deadline to re-enroll. "How do you expect me to pay that?'"
Experts say online applications and electronic verification of income levels and other things will make this process easier. But deadlines will still matter and some people don't have easy access to the Internet. And there will still be some people who simply won't enroll.
"There will always be that segment that says, 'Aw, the heck with it, I will just wait until I get sick and go to the ER,'" says Stephen Schilling, CEO of Clinica Sierra Vista, a nonprofit that has a network of 27 community health centers in California.
Schilling expects to still see a lot of uninsured patients at the nonprofit group's health centers even if the law is upheld. The center sits in an agricultural area in California's San Joaquin Valley, populated with migrant workers and saddled with an unemployment rate of around 15 percent.
It cares for about 60,000 uninsured people annually, thanks in part to grants and a sliding fee scale for patients based on their family size and income. Schilling says he still expects between 20,000 and 40,000 uninsured patients if the overhaul is implemented.
LIVING IN THE GAP • The overhaul calls for tax credits to help middle-class Americans buy coverage. But some people who make too much money to qualify for the tax credits may have a hard time finding an affordable option for private health insurance
The subsidies can pay a large chunk of the insurance bill. For instance, a 40-year-old person who makes $50,000 in 2014 and needs coverage for a family of four might receive a government tax credit of more than $8,000.
That would cover more than 70 percent of the premium, or the cost of coverage, according to a subsidy calculator on the nonprofit Kaiser Family Foundation's website. Of course, that estimate depends on the type of coverage the person choses, where they live and whether they can get coverage through work.
But the tax credits will go to people with incomes up to 400 percent of the federal poverty level, or $44,680 for an individual this year. People just above that level may have a hard time finding affordable health insurance.
Angela Agnew Laws worries that she might remain uninsured like she has been for the past eight years even if the health care law is upheld.
Laws, who lives in Leesburg, Va., runs a small business that cleans and maintains commercial buildings. She hopes her income will climb to about $60,000 by 2014, which would be too high for tax credit help.
A plan that offers more than just basic protection against big medical expenses could cost as much as $10,000 annually for Laws. She could find less extensive coverage for a lower premium, but that may only save about $1,000.
Laws, 58, figures that she'll remain uninsured if she can't find an affordable coverage option that fits a monthly budget already crammed with payments of $1,203 for rent $530 toward her car.
"It's a scary prospect for me," she says.