Even as it faces another Supreme Court challenge, President Barack Obama’s health care law has steadily reduced the number of uninsured Americans, according to an extensive survey released Tuesday.

The Gallup-Healthways Well-Being Index found that the share of adults without health insurance dropped to its lowest level in seven years in 2014 as Obama’s overhaul took full effect.

The trend appears likely to continue, since 55 percent of those who remained uninsured told the pollster they plan to get coverage rather than face rising tax penalties.

No state saw a statistically significant increase in the share of its residents uninsured. However, states that have embraced the health care law continued to outperform those where political opposition remains strong.

Ten of the 11 states with the biggest declines in their uninsured rates accepted the health care law’s Medicaid expansion, which provides safety-net coverage for low-income residents, mainly adults with no children living at home. Topping the list were Arkansas and Kentucky, with double-digit decreases.

Montana, in the midst of an oil boom, was the exception among the top performers. Its uninsured rate dropped by 4.9 percentage points from 2013 to 2014, but that state has not expanded Medicaid.

In addition to expanded Medicaid, Obama’s law offers subsidized private coverage for people who don’t have access to it on the job. The federal government is running the online insurance markets, or exchanges, in most states. But the top-performing states — except for Montana — are either running their own insurance markets, or partnering with the federal government.

The Gallup-Healthways report comes at a time of high uncertainty over the health care law. Next week, the Supreme Court will hear arguments in a case that challenges insurance subsidies for residents of states that have not set up insurance exchanges, defaulting instead to federal management. The federal HealthCare.gov currently serves 37 states.

Opponents of the law argue that its literal wording only allows subsidized premiums in states that have set up their own exchanges. Supporters say that’s a narrow reading that ignores lawmakers’ clear intent to create a national program.

The Gallup survey is considered authoritative because it combines the speed of media polls with the thoroughness of large government studies. Pollsters interview 500 people a day, 350 days a year. In 2014, that added up to nearly 177,000 adults aged 18 and older.

Comparing full-year results, Gallup found a drop of 3.5 percentage points in the share of Americans without health insurance from 2013 to 2014.

The five most populous states all saw declines. For California, it was a drop of 6.3 percentage points; Texas, 2.6 points; Florida, 3.8 points; New York, 2.5 points and Illinois, 4.5 points.

Texas remained the state with the highest uninsured rate, at 24.4 percent, while Massachusetts had the lowest, at 4.6 percent. Massachusetts had expanded coverage earliest, under former GOP Gov. Mitt Romney, whose approach foreshadowed some of the central elements of Obama’s law.

Dan Witters, research director for the poll, said about 12.3 million fewer adults were uninsured in the last three months of 2014, when compared with the third quarter of 2013, when HealthCare.gov was launched.

The survey has a margin of sampling error of plus or minus 1 to 2 percentage points for most states.

(AP)