The open enrollment period for people to sign up for health insurance next year through the Affordable Care Act ends Saturday in most states, and signups are significantly down from last year.

While President Donald Trump didn't repeal and replace former President Barack Obama's signature health-care law, more commonly known as Obamacare, he was able to dismantle key parts of it that health policy researchers are blaming for much of the drop.

Sign-ups on the federal health insurance marketplace have fallen 11.7 percent from the same time last year, according to the latest figures from Centers for Medicare and Medicaid Services. The number of new people buying the coverage has dropped even more: 19.7 percent.

The lagging numbers also come as the future of the Affordable Care Act is uncertain. A federal judge in Texas ruled late Friday the law unconstitutional, potentially threatening health-care coverage for millions of Americans and setting up a new legal showdown over Obama's policy initiative. The lawsuit was backed by the Trump administration, and is likely to be appealed — which could mean the legislation will heard anew by the Supreme Court, which upheld Obamacare in a narrowly divided 2012 ruling.

Trump cheered the judge's decision in a tweet.

As I predicted all along, Obamacare has been struck down as an UNCONSTITUTIONAL disaster! Now Congress must pass a STRONG law that provides GREAT healthcare and protects pre-existing conditions. Mitch and Nancy, get it done!

To be sure, the tight labor market is playing at least some role in lower Obamacare enrollment figures this season, health policy experts say.

Historically low unemployment, which was at 3.7 percent in November and October, is helping reduce dependence on the federal health program as more Americans are getting their health insurance from employers. And while Obamacare doesn't require small businesses to offer health coverage, more may now be doing so to attract and retain workers.

The ACA's final enrollment numbers won't be tallied until next week, but health policy experts say several key changes the Trump administration made to the ACA law are helping drive enrollment down this year.