WASHINGTON — 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 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.