Going into President Barack Obama’s last year in office, progress has stalled on reducing the number of uninsured Americans under his signature health care law, according to a major survey out Thursday.

The share of U.S. adults without health insurance was 11.9 percent in the last three months of 2015, essentially unchanged from the start of the year, according to the Gallup-Healthways Well-Being Index. The ongoing survey, based on daily interviews with 500 people, has been used by media, social scientists, and administration officials to track the law’s impact.

Release of the latest installment comes after the Republican-led Congress voted to send legislation repealing the Affordable Care Act to Obama’s desk. The president is certain to veto it, but opponents say that will only help their strategy of keeping “Obamacare” alive as a political issue in the presidential election.

The sharp drop in the uninsured rate seen in 2014 — the first year of the law’s major coverage expansion — now has leveled off, Gallup said in its analysis of the latest findings.

“This validates concerns that similarly large reductions may not be possible in the future because the remaining uninsured are harder to reach or less inclined to become insured,” the analysis said. “Future reductions will likely require significant outreach and expanded programs targeting those who have not yet taken advantage.”

The survey period included the first two months of the health law’s 2016 open enrollment season, which ends Jan. 31. That covered the first big sign-up deadline, which was Dec 15 for those wanting coverage effective at the start of the year.

Gallup-Healthways said it expects results for the first three months of 2016 to show another decline in the uninsured rate. How big remains to be seen.

“A strong open enrollment period would allow the Obama administration to go out on a high note,” said Larry Levitt, who follows the health overhaul for the nonpartisan Kaiser Family Foundation. “Weak enrollment could intensify the debate over the Affordable Care Act, particularly as the general election approaches,” he said.

“There will continue to be close scrutiny of the uninsured numbers and reports of premium increases for 2017, which will start to trickle out this summer,” added Levitt. “I believe the health law has crossed the threshold of sustainability, but its future success depends on growing enrollment.”

The uninsured rate for adults stood at 17.1 percent in the last three months of 2013, as the law’s major coverage expansion got underway, according to the survey. The drop of 5.2 percentage points by the end of last year translates to nearly 13 million adults gaining coverage.

But 2015 was a lackluster year. According to the survey, the uninsured rate dipped to 11.4 percent in the period from April-June, then edged up again the rest of the year. It was the first such reversal noted by the survey since the law’s big coverage expansion began.

The health law has added coverage in two major ways, bringing the nation’s uninsured rate to a historic low. Online insurance markets like HealthCare.gov offer taxpayer-subsidized private plans to people who don’t have coverage on the job. And states can opt for a Medicaid expansion aimed at low-income adults with no children living at home. That’s happened in 30 states, plus Washington, D.C.

Opposition to the law has blocked Medicaid expansion elsewhere. If major states like Texas, Florida, Georgia, North Carolina, and Virginia were to expand their programs, the uninsured rate would start coming down again. Louisiana and Alabama are considering Medicaid expansion.

Since 2013, the Gallup-Healthways survey has found gains in health insurance coverage among all major demographic groups except seniors, who were already covered by Medicare. The biggest progress has come among low-income people, Hispanics, and African Americans.

The Obama administration had no direct response to the survey findings. Spokesman Aaron Albright noted that the law “has led to millions of Americans getting access to quality and affordable health coverage.”

The survey results were based on landline and cellphone interviews conducted from Oct. 1 to Dec. 31 with a random sample of 42,998 adults ages 18 and older. For results based on the total sample, the margin of sampling error is plus or minus 1 percentage point.

(AP)