This group is more likely to live in poverty and is only one subset of the underinsured, because the report does not include insured people who needed care and went without it because of the out-of-pocket costs they would face, the report's authors said.
"[It's] exactly the people the Affordable Care Act was designed to help," said Commonwealth Fund senior vice president Cathy Schoen, lead author. "However, if all states don't expand Medicaid, millions will still go without health insurance and health care."
In 2012, there were 32 million insured people under age 65 who were underinsured, the study shows. Together with the nation's 47 million uninsured, this means at least 79 million are at risk for not being able to afford needed care.
The Affordable Care Act has, so far, covered 17.3 million since its inception in 2010.
There is debate about the quality of health coverage sold on the federal insurance marketplace, www.healthcare.gov. Health plans are categorized by metal tiers, and bronze level plans, while affordable up front, cover only 70 percent of policyholders' medical bills. There's also talk of enhancing benefits.
Regardless, the health law's ability to chip away at the uninsured and underinsured problem "will depend critically on state decisions regarding Medicaid," the report says.
Utah leaders have been slow to warm to the idea of expanding Medicaid to cover more of the state's poor and uninsured. Gov. Gary Herbert is negotiating with the Obama administration to get a block grant for Utah's share of federal expansion dollars. It would be used to buy private coverage for those with incomes under 138 percent of the poverty level, such as a single person earning under about $15,000.
But it's not clear if his fellow Republican leaders in the House and Senate support the idea.