Most uninsured Americans are sitting on the sidelines as sign-up season under the federal health law comes to a close, according to a new poll that signals the nation’s historic gains in coverage are slowing.

The survey released Thursday by the nonpartisan Kaiser Family Foundation finds that:

— Only 15 percent of the uninsured know this year’s open enrollment deadline, which is Sunday.

— More than 7 in 10 say they have not tried to figure out if they qualify for the two main coverage expansions in the law, Medicaid and subsidized private health insurance.

— Only 1 in 100 know the minimum penalty for being uninsured is going up to $695 in 2016.

— About two-thirds say they have not been contacted about signing up for coverage.

“What this survey does suggest is that it will get harder and harder to continue to make gains in the share of people getting health insurance,” said Mollyann Brodie, Kaiser’s polling director. “I think we will be in a period where we will see slower and slower gains in that number.”

President Barack Obama’s health care law has driven down the share of uninsured Americans to 9 percent, according to the government. Now, in the third year of the law’s major coverage expansion, those historic gains could be stalling out. That’s raising questions about the lasting impact of Obama’s signature legislation, and spurring liberals like Democratic presidential candidate Bernie Sanders to call for guaranteed coverage for all.

The health law’s problems in part reflect entrenched political opposition, but there’s also skepticism about whether the coverage is affordable for uninsured people of modest means. Premiums are up, and those who get sick can face significant out-of-pocket costs. Major health insurer Anthem said Wednesday it booked fewer customers than expected last year through the health care law.

The poll found that nearly half the uninsured (46 percent) have been without coverage for two years or longer.

The Kaiser poll is the second independent survey to raise questions recently. Earlier this month, the Gallup-Healthways Well-Being Index found that the law’s progress stalled last year. The share of U.S. adults without health insurance was 11.9 percent in the last three months of 2015, according to Gallup, essentially unchanged from the start of the year.

The health law has added coverage in two major ways. Online insurance markets like HealthCare.gov offer taxpayer-subsidized private plans to people who don’t have coverage on the job. Sunday is the marketplace sign-up deadline.

The Obama administration has set a modest goal of 10 million people enrolled through the markets and paying premiums at the end of the year. Earlier this week, the Congressional Budget Office reduced its own enrollment projections.

The law’s other big driver of coverage is a Medicaid expansion aimed at low-income adults with no children living at home. Thirty-one states, plus Washington, D.C., have opted to expand. But populous Republican-led states including Texas, Florida, Georgia and North Carolina have turned down the federal offer. If they were to accept, the coverage expansion would regain its momentum.

The poll found that Americans remain divided on the Affordable Care Act, with 44 percent viewing it unfavorably and 41 percent holding a favorable view.

The Kaiser survey was conducted Jan. 13-19 among a nationally representative sample of 1,204 adults. For the entire poll, the margin of sampling error is plus or minus 3 percentage points. For the uninsured, the margin of sampling error is plus or minus 10 percentage points.

(AP)