WASHINGTON — More than 8.8 million Americans signed up for Affordable Care Act health insurance plans for 2018 in the 39 states relying on the federal HealthCare.gov website, an unexpectedly robust turnout that approaches the 2017 total, despite an enrollment season cut by half and other moves by the Trump administration to undermine the law’s insurance marketplaces.

The numbers that the administration reported Thursday include an all-time high for the number of new consumers signing up in a single week, with 1 million such people picking health plans in the final days before the Dec. 15 federal deadline.

The overall tally is roughly 95 percent of the 9.2 million people who got coverage for 2017 during a three-month sign-up period that ended in January. It defies widespread expectations of the law’s critics as well as supporters that enrollment would slump sharply as the Obama administration’s boosterism gave way to the aggressive opposition of President Trump and congressional Republicans, who have tried all year to repeal much of the ACA.

Federal health officials withheld the total for more than 24 hours past their usual release day and then made it public in atypical fashion. Seema Verma, administrator of the Health and Human Services Department’s Centers for Medicare and Medicaid Services tweeted the single figure. Two hours later, the snapshot became public, but it contained inconsistencies.

For the seventh and final sign-up week, ending on Dec. 15, the report said, 4.1 million people had signed up for coverage or been automatically renewed by the government because they had ACA health plans this year and had not selected ones for 2018. However, federal health officials had previously said the automatic renewals would not happen until after the federal marketplace’s enrollment deadline. Asked for a breakdown between the active sign-ups and the auto-enrollments, officials refused to provide it.

Even with some information obscured, the snapshot reflects an intense surge of consumers rushing to get coverage before it was too late. The 1 million new customers who picked health plans last week compares with a previous one-week record of just under 900,000 two years ago.

The tally does not include people signing up in states that run their own insurance marketplaces under the law or in several states in the federal exchange where enrollment seasons were extended until late December because they were affected by hurricanes and other major fall storms. Every year, some people who pick health plans or are automatically renewed do not end up with coverage because they never start paying monthly premiums.

The unexpected popularity of the ACA’s marketplaces at a time of intense political turmoil surrounding the 2010 law drew swift praise from its proponents. The enrollment tally so far “makes it crystal clear that Americans demand and support the quality, affordable health insurance and consumer protections the ACA offers,” said Robert Restuccia, executive director of Community Catalyst, a large grass roots health-care advocacy group.

“This is a remarkable result given the Administration’s efforts to sabotage enrollment by gutting outreach, creating chaos and confusion, cutting off subsidies for low-income families and shortening the enrollment period,” Rep. Frank Pallone Jr., D-N.J., the ranking Democrat on the House Energy and Commerce Committee, said in a statement.

Beyond shortening the enrollment period by half, the administration cut by 90 percent federal spending on advertising and other outreach activities to urge consumers to sign up. It cut funding for enrollment “navigators” by about two-fifths. Surveys and on-the-ground reports of those enrollment helpers suggested public confusion over whether the law and its marketplaces still existed.

In a statement accompanying the enrollment snapshot, Verma included a justification for the funding cuts. “Our goal from the beginning was to empower patients across the healthcare delivery system and make sure that Americans who chose to enroll in the Exchanges had a good customer experience while making enrollment more cost efficient, and the results show that we accomplished our goal,” she said.

“In a market that is experiencing soaring rates,” she continued, “I am proud of the hard work CMS put into making sure that our customers didn’t experience the website failures that were commonplace with HealthCare.gov in previous open enrollment periods.”

The federal enrollment website debuted the fall of 2013 with serious defects that stymied many consumers. But it has been largely trouble-free the past three years, and consumers and enrollment helpers this season reported only scattered technical problems.