Unexpectedly strong enrollment on the federal health insurance exchange means the total number of Obamacare sign-ups this season could come close to or even match the prior season's tally, despite serious headwinds.

And that will make it even harder than it already has been for the Trump administration to undo Obamacare, advocates for that health-care law say.

On Thursday, Trump health officials said at least 8.8 million people so far had signed up on the federal insurance marketplace, HealthCare.gov, by last Friday's open-enrollment deadline, for coverage in 2018.

That was only about 400,000 fewer than had enrolled in individual plans on that exchange at the close of the previous sign-up season for 2017 coverage. And more than 4 million people signed up during the final week of enrollment on HealthCare.gov, which was a record.

"I think, overall, people were surprised at just how good the numbers were," said Lori Lodes, a former health official in the Obama administration. "It's just another proof point that people don't want their health care taken away."

Lodes and other Obamacare experts had previously expected that final enrollment on HealthCare.gov, which serves 39 states, would be markedly lower this season than it turned out to be for several reasons.

Among them was the fact that the enrollment window for HealthCare.gov this year ran only from Nov. 1 through Dec. 15 — half the amount of time that customers were given last year. Polls showed that many people were unaware of the earlier deadline.

Another factor that was expected to depress enrollment was sharp cutbacks by the Trump administration in advertising and outreach efforts designed to spur people to sign up for Obamacare plans. The administration also has consistently criticized the law and sought its repeal, leading some people to think it no longer remained in place.

"I was shocked" by HealthCare.gov's tally, said Larry Levitt, an Obamacare analyst with the Kaiser Family Foundation.

Levitt noted that because of the earlier deadline and outreach cutbacks, he had previously expected national enrollment to be 1 million or 2 million fewer than the 12.2 million who enrolled by the close of open enrollment last season.

National enrollment includes the tallies of both HealthCare.gov and the 12 other Obamacare exchanges run by individual states and the District of Columbia.

But because of HealthCare.gov's stronger-than-expected showing, "It is possible enrollment could match or even exceed last year" nationally, said Levitt.

"I find that amazing," Levitt tweeted Friday.

Levitt's revised outlook stems from the fact that enrollment on HealthCare.gov will actually be higher than what was reported on Thursday, and that sign-ups on a number of state-run exchanges that have yet to close enrollment are running higher than last year.

Thursday's reported tally for HealthCare.gov did not include people who signed up during the three hours after midnight Dec. 15. Open enrollment on the exchange officially closed at 3 a.m. ET last Saturday.

Nor did the tally include people who were in line on the federal sign-up site at the time the deadline passed, but who had left their contact information so their enrollments could be completed at a later date.

Lodes, the former Obama administration official, said she expects another 50,000 or more enrollments could come from those two groups of people.

Another 50,000 or more could come from people in HealthCare.gov states who are being allowed to enroll through Dec. 31 because they lived in areas affected by hurricanes or unusually high winds this year, Lodes said. The entire state of Alabama, Florida, Georgia, Maine and South Carolina, and parts of Mississippi, Louisiana and Texas are covered by that special enrollment waiver.

With those people factored in, Lodes said enrollment on HealthCare.gov could reach 9 million or so.

Yet to be added to HealthCare.gov's tally is the final enrollment totals from state-based markets, all but two of which had later deadlines than the federal marketplace this year.

Earlier this week, the nation's biggest state-run market, California, which has a Jan. 31 deadline, said that more than 220,000 new customers had enrolled, about 10 percent more than last year during the same time period.

And about 1.2 million existing Covered California customers have had their coverage renewed for 2018.

Washington state's exchange, which has a Jan. 15 deadline, said it has already booked 230,000 plan selections — a 35 percent increase over the same time last year.

"The updated total also eclipses the record 225,000 sign-ups documented at the close of the previous enrollment period," the exchange said in a prepared statement.

And selections by new customers of the Washington exchange are up 89 percent over 2016.

Minnesota's exchange said enrollments are running 12.5 percent higher, with more than 108,500 plan selections. The sign-up deadline in Minnesota is Jan. 14.

Colorado's exchange, whose deadline is a day after, said that 149,000 people so far have enrolled, a 7 percent increase over the same time last year.

Charles Gaba, who runs the Obamacare tracking site ACASignups.net, said, "I've confirmed 2.8 million [enrollments] on the state exchanges" so far this season, "and it was 3 million last year."

"I expect them to do that or come extremely close" this year, Gaba said of the state markets.

That would mean national enrollment would be at least 11.8 million, and possibly even closer to last season's tally of 12.2 million, once HealthCare.gov's extra sign-ups are factored in.

"I'm pretty sure it's going to come extremely close or possibly break even," Gaba said.

Gaba earlier this year had said national Obamacare enrollment could end up being as low as 10 million because of the shorter deadline and antagonism toward the program by the Trump administration. But Gaba also had said enrollment could be as high as 14 million because "it was a crapshoot this year."

Oddly, a Trump administration action to undercut Obamacare might have actually boosted enrollment higher than it otherwise would have been this year.

President Donald Trump for months after taking office had threatened to stop reimbursing Obamacare insurers for discounts those insurers by law must give most customers in their out-of-pocket health costs.

Trump followed through on that threat in October — but at that point many insurers had already factored in the loss of the payments by raising premiums higher than they originally planned.

The higher prices likely dissuaded some customers from signing up, particularly those customers who do not qualify for federal subsidies that reduce how much they personally pay in premiums.

But the price hikes also meant that subsidized customers — who have low and moderate incomes — saw their subsidies increase in value.

As a result, more Obamacare customers than ever were able to find plans that personally cost them $0 per month after their subsidies were factored in.

Levitt said the relatively strong enrollment seen this year means "we can put aside concerns" that Obamacare insurers would see their so-called risk pools deteriorate, and have to raise prices in response next year. A risk pool is the mix of customers, and weak risk pools have too many sick customers and not enough healthy ones.

Without sharply higher prices forecast for 2019, and with steady enrollment, Levitt said the Trump administration will find it more difficult to repeal and replace large parts of Obamacare than it already experienced this year.

"It's easier to attack and repeal a program that's on the decline, so strong enrollment numbers make the law much harder to roll back," Levitt said.

On Friday, Trump signed a tax bill that starting in 2019 will suspend enforcement of Obamacare's individual mandate, which requires most Americans to have some form of health coverage or pay a tax penalty.

The Congressional Budget Office has projected that 13 million more Americans could be uninsured by 2027 as a result of that move.

Levitt said he believes that projection is too high. He said there is good reason to believe that many people are signing up for health insurance not because they are forced to, but because they want to and can now afford to.

He said that the availability of premium subsidies for most Obamacare customers — about 85 percent of exchange customers get such aid — has proven to be a big driver of enrollment.