The second major ACA program was the expansion of Medicaid eligibility to nearly all people whose income is below 138 percent of poverty.
A 2012 Supreme Court decision said that states could not be compelled under the ACA to expand Medicaid. But so far, 28 states and D.C. have expanded Medicaid, with Montana's proposed expansion awaiting federal approval.
Most of the people whose states have not expanded Medicaid live in the South.
Since Obamacare, private insurance exchanges began selling plans in 2014, and as Medicaid expansion took effect in more states, about 25 million have gotten coverage through one of these two provisions, as well as through another ACA program that allows people up to age 26 to stay on their parents' health plans.
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As a result, "the uninsured rate has dropped across all states, regardless of whether they have expanded Medicaid or not," Tolbert said.
But, she added, "we've seen a much bigger drop in the uninsured rates among the states that have adopted Medicaid expansion."
State-by-state uninsured-rate data are not yet available, although it is expected to be released later this year.
At the same time that the overall uninsured rate dropped, so did the costs of providing care to people who lacked insurance.
But again, there was a significant difference in outcomes between the states that expanded Medicaid and those that didn't.
Last month, the federal government said "uncompensated care costs" that hospitals nationally incurred in 2014 were $7.4 billion lower than they would have been if insurance coverage had remained at 2013 levels.
However, the lion's share of those savings—$5 billion—came from states that expanded Medicaid.
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A report issued in late April by the Kaiser Family Foundation looked at the experience of Ascension Health, which operates 131 acute-care hospitals and more than 30 senior-care facilities. Ascension's facilities are located in the District of Columbia and seven states that expanded Medicaid, along with nine states that did not expand.
"Overall, [Ascension] hospitals in Medicaid expansion states saw increased Medicaid discharges, increased Medicaid revenue and decreased cost of care for the poor, while hospitals in nonexpansion states saw a very small increase in Medicaid discharges, a decline in Medicaid revenue and growth in cost of care for the poor," the report said.
In a line item that contributed to that finding, Ascension hospitals in expansion states saw a 40.1 percent decrease in charity-care costs alone. That compares with a drop of just 6.2 percent in charity-care costs for Ascension facilities in nonexpansion states.