Obamacare remains pretty popular with many Americans, despite President Trump and Republicans' repeated attempts to kill it.

Some 8.8 million people have signed up for 2018 coverage on the federal exchange during an open enrollment season that was half the length of prior years and far less promoted, the Trump administration said Thursday. That's only 400,000 fewer than signed up on healthcare.gov during open enrollment a year ago.

Nearly 2.4 million consumers were new to the exchanges, while more than 6.4 million continued their coverage during the period, which ran from Nov. 1 through Dec. 15.

Traffic was heavy in the final days. More than 4.1 million people selected plans in the last six days, including those who were automatically renewed.

Unlike in prior years, the Trump administration didn't extend the enrollment deadline, though it did give people who couldn't get through to the call center a little more time to sign up. These folks were not included in Thursday's figures.

Exchange open enrollment for 2018 coverage ended w/ approx 8.8M people enrolling in coverage. Great job to the @CMSGov team for the work you did to make this the smoothest experience for consumers to date. We take pride in providing great customer service. — CMS Administrator (@SeemaCMS) December 21, 2017

The final numbers for the federal exchange, which handles enrollment for 39 states, is likely to grow more. Those affected by the hurricanes in several Southern states, as well as Puerto Rico and the U.S. Virgin Islands, and those who had their plans discontinued by their insurer also have a little more time to sign up. The former have until the end of the year, while the latter have until March 1.

Enrollment proved strong throughout the 45-day enrollment period, though it hasn't yet matched the 9.2 million who picked policies during last year's period, which ran until Jan. 30.

Obamacare supporters and health care experts cheered the surprisingly strong showing.

"The demand for affordable coverage speaks volumes -- proving, yet again, the staying power of the marketplaces," said Lori Lodes, a former Obama administration official and co-founder of Get America Covered, which seeks to promote open enrollment.

The Trump administration made several changes to this year's open enrollment period that supporters of the Affordable Care Act said sabotaged the program. In addition to shortening the enrollment period, it reduced the advertising budget by 90%, eliminating television spots, which advocates said was the most effective way to get people to sign up. Also, the administration cut funding for navigators, who help people enroll, by more than 40%.

The Trump administration sought to cast a positive spin on the enrollment season, noting it spent far less on advertising for nearly the same number of sign ups.

This year CMS took a more cost effective outreach approach, spending just over $1 per enrollee on outreach and education for Exchange coverage compared to nearly $11 per enrollee last year. — CMS Administrator (@SeemaCMS) December 21, 2017

"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," said Centers for Medicare and Medicaid Services Administrator Seema Verma.

Related: Obamacare shoppers find fewer insurer choices on exchanges

The final tally of 2018 enrollment won't be known for a few more months. Several states that run their own exchanges are giving their residents more time to sign up and have reported that business is brisk. California and New York, for instance, are letting consumers enroll through January 31. Minnesota said sign-ups are running 12.5% ahead of this time last year, while 10% more New Yorkers picked plans. Washington, meanwhile, saw sign-ups soar by nearly 35% through Dec. 15.

Some three million people selected 2017 policies in state-based exchanges during open enrollment for 2017, bringing the total to 12.2 million.

Enrollment isn't final until consumers pay their first month's premium. Many people also enter or exit the individual market during the year because of certain situations, such as getting a job or losing one. Some 10.1 million people were enrolled in Obamacare plans at the end of June, about 2.1 million fewer than had selected plans when open enrollment ended in January.

Related: Will Obamacare survive the tax bill?

Looking ahead to 2018, enrollment is a key indicator of the health of the exchanges. A decline in popularity may signal that fewer younger and healthier people signed up, leaving mostly older and sicker people in the marketplace.

This is exactly what insurers fear. They'll be keeping a close eye on the health care bills their members run up next year. If expenses soar, they may think twice about staying in the exchanges in 2019.1

Compounding their concerns: Congress is eliminating the individual mandate, which requires most Americans to have insurance or pay a penalty. This is also expected to tilt the individual market to older and sicker consumers. The provision would take effect in 2019.