The study, one of a series on the fractured state of the $2.6 trillion U.S. healthcare system published in the May issue of the journal Health Affairs, says access to health care deteriorated for U.S. adults aged 19 to 64 between 2000 and 2010, even among those with private health insurance.

The age group represents about 195 million people, according to U.S. Census data, and has been targeted for expanded health coverage under the 2010 Patient Protection and Affordable Care Act, Obama's signature domestic policy achievement.

The Supreme Court is considering whether to strike down all or part of the law, with a ruling due next month.

The law would extend affordable health coverage to more than 32 million uninsured Americans beginning in 2014, by creating subsidized, state-regulated health insurance markets and by expanding the joint federal-state Medicaid program for the poor.

The U.S. Department of Health and Human Services says access to care and use of healthcare services has declined in recent years, partly as a result of a recession that swelled the ranks of the unemployed and uninsured.

Researchers at the nonpartisan Urban Institute found the deterioration in access to care was evident even earlier, from 2000 to 2010, as spiraling healthcare costs led to reductions in employer-sponsored insurance benefits and strained the existing Medicaid system for the poor.

An exception was seen for children, who were protected from the decade-long erosion by concerted efforts to enhance their coverage. On that basis, the researchers predicted that adults would benefit from reforms that counter higher costs for the privately insured, raise Medicaid payments to providers and shore up the social safety net.

"If the key coverage provisions in the (law) are ruled unconstitutional or repealed, projections indicate that the numbers of uninsured people will grow," the researchers wrote.

"Given what we have observed over the past decade, we would be likely to see further deterioration in access to care for all adults -- insured and uninsured alike," they said.

STATE-BY-STATE SUCCESS?

The researchers found that adults in 2010 were 66 percent more likely to report unmet medical needs than in 2000, 79 percent more likely to have unmet dental needs and were also more likely to have delayed treatment.

The deterioration of access was worst among the uninsured. But by 2010, 10.2 percent of Americans with private insurance reported having unmet medical needs, vs 5.8 percent in 2000. The number of privately insured who delayed care because of costs also climbed, to 6.8 percent from 3.9 percent.

The Health Affairs studies also examine prospects for reduced racial and geographical disparities in healthcare, appearing during a rancorous election-year debate in which Obama's political allies and opponents are vying to control the healthcare message to voters.

Republicans, who hope to capture the White House and the Senate in November, are appealing to voters who dislike the healthcare law by pledging to repeal it.

The healthcare law's success could depend on how well its provisions, including Medicaid expansion, are supported by state governments, more than half of which currently oppose the law.

Healthcare policy experts at Harvard and the Dana-Farber Cancer Institute in Boston found that Medicaid participation nationwide currently stands at 63 percent of eligible adults.

Results range between by states, from 43 percent in Arkansas and Louisiana to 83 percent in Massachusetts, which enacted reforms similar to Obama's law in 2006.

Their research warned that the take-up rate for expanded Medicaid coverage in 2014 could be lower than anticipated if benefits prove to be restrictive.

"To encourage high participation in the expanded Medicaid program, states will need to offer comprehensive coverage of needed benefits, provide community-based outreach and consider more dramatic changes to their enrollment processes, such as automatically enrolling people in Medicaid based on their participation in other public programs," the experts said.

The Medicaid expansion and creation of new insurance exchanges would greatly reduce long-standing racial and ethnic disparities in access to care demonstrated by data that currently show uninsured rates to be 2.6 times higher among Hispanics and 1.8 times higher for blacks, compared with non-Hispanic whites, according to another Urban Institute study.

But the degree of improvement, especially for Hispanics, will depend on how state governments respond.

"Coverage gains among Hispanics will probably depend on adoption of strategies that address language and related barriers to enrollment and retention in California and Texas, where almost half of Hispanics live," researchers said.

(Editing by Michele Gershberg and Todd Eastham)

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