WASHINGTON (AP) — Eight states saw a significant drop last year in the number of residents going without health insurance, according to a government report out Tuesday that has implications for the presidential campaign.

All but Florida had accepted a Medicaid expansion that is one of two major pathways to coverage under President Barack Obama's health care law. The law's other coverage route is subsidized private insurance, available in all 50 states.

GOP presidential candidates are vowing to repeal "Obamacare," while offering hardly any detail on how they'd replace it without millions losing coverage.

Politically, the eight states with statistically significant coverage gains in the National Health Interview Survey are a mix of red, blue and purple. They are Arizona, California, Colorado, Florida, Illinois, Kentucky, Michigan, and New York. Five have GOP governors.

As a whole, the nation had an uninsured rate of 9.1 percent during the first nine months of 2015, according to the survey, an ongoing research project by a unit of the Centers for Disease Control and Prevention. The uninsured rate was 14.4 percent in 2013, before the law's big coverage expansion.

The federal report does not analyze the reasons for the coverage gains, but independent experts say the trend is due to the Obama health care law, boosted by economic recovery.

That poses a dilemma for Republican presidential candidates. Indeed, a recent blueprint from a group of conservative policy experts for replacing the health law said Republicans will need some kind of "grandfathering exemption" to avoid disrupting the lives of people who have gained coverage through the Affordable Care Act.

The new state numbers from the National Center for Health Statistics focus on adults ages 18-64. That's because virtually all seniors have coverage, and fewer than 5 percent of children and teens are uninsured.

In addition to the eight states with statistically significant coverage gains, the report named another 10 with notable reductions in the percentage of uninsured residents. However, the changes in these states did not meet the survey's test for statistical significance.

That second group included Georgia, Idaho, Indiana, Louisiana, Mississippi, New Hampshire, New Mexico, North Carolina, Oklahoma, and Rhode Island. Seven of those states have Republican governors.

Kentucky led the way among the states with statistically significant reductions in the share of residents uninsured, with a drop of 6.5 percentage points. The state's new Republican governor, Matt Bevin, ran as a strong critic of Obama's law and is now grappling with how to scale back his state's involvement.

Another Republican-led state, Arizona, had the second-biggest reduction, a drop of 5.9 percentage points. New York followed, with a reduction of 5.6 percentage points.

The health care's law big coverage expansion got under way in 2014, and since then the nation's uninsured rate has fallen to a historically low level. With the 2016 sign-up season recently concluded, the Obama administration is hoping to see continued improvement this year.

The law offers subsidized private health insurance through online markets like HealthCare.gov for people who don't have access to employer-based coverage. That's coupled with a Medicaid expansion aimed at low-income adults, now accepted by 31 states. Virtually all Americans are required to have coverage or risk paying fines to the IRS.

Even with Obama's law, nearly 29 million people were still uninsured in the nine months from Jan.-Sept. of 2015. That includes an estimated 11 million immigrants without legal permission to be in the country, who are not entitled to coverage.