Wait times for the federal Obamacare marketplace's call center were already relatively long early Friday, the final day for enrollment in individual health plans in 41 states.
"Like in previous years, we have seen an uptick in the volume of consumers on HealthCare.gov and contacting our call center," said a spokeswoman for the federal Centers for Medicare and Medicaid Services, which runs that Obamacare exchange.
"As a result, some callers are being asked to leave their contact information so they do not have to wait. We will call them back to assist them."
All HealthCare.gov customers who leave their contact information will be able to enroll in an Obamacare plan that takes effect Jan. 1, even if the application is not actually completed by Friday.
The HealthCare.gov online enrollment site "is performing well and consumers can easily access enrollment tools to compare plans and prices," said the CMS spokeswoman.
People who do not sign up by the coverage deadlines cannot enroll in an individual health plan during 2018 unless they qualify for certain exemptions. And Americans who lack health insurance risk paying a tax penalty.
Get America Covered, an Obamacare advocacy group, said wait times at the call center became lengthy earlier in the week, as the sign-up deadline approached.
As of 7:45 a.m. Friday, "Waits at call center are over 30 minutes ... which is incredibly long for this early," said Lori Lodes, a former Obama administration health official and one of Get America Covered's founders.
Lodes said that a Google Trends analysis showed that visits to HealthCare.gov's website "exploded" on Thursday, "hitting its highest point at midnight."
Other states that run their own Obamacare marketplaces have later deadlines this season.
California's exchange, whose final enrollment deadline for 2018 coverage is Jan. 31, on Thursday said it had seen "a strong surge in demand" this week on the marketplace, the largest state-run Obamacare site.
"Over the past three days, Covered California has seen more than 38,000 new consumers sign up for coverage, which means that as of Dec. 13 more than 182,000 new consumers have signed up for coverage since open enrollment began, that marketplace said.
"Compared to the same time last year, when 156,000 had signed up, this enrollment marks an increase of 26,000, or 17 percent."
In response to the demand, Covered California has extended, by one week, its deadline for signing up for a health plan that goes into effect on Jan. 1.
Californians will now have until Dec. 22 to sign up for coverage that starts New Year's Day.
The enrollment deadline comes as Congress considers a tax bill that, among other things, would effectively repeal the Obamacare rule that requires most Americans to have some form of health coverage or pay a fine.
It also comes as the Trump administration has slashed spending on advertising and outreach efforts designed to encourage people to sign up for Obamacare plans and has refused to let federal health officials participate in enrollment promotion activities.
Lodes, the Obamacare advocate, and other supporters of the health-care law have predicted that as a result of those actions, enrollment in individual health plans will be lower for 2018 than it was for 2017.
A total of 12.2 million people had selected an Obamacare plan when open enrollment for this year ended last winter.
The pace of enrollment on HealthCare.gov this sign-up season, which began Nov. 1, has actually been faster than the pace seen last season.
However, the sign-up deadline on the federal site is six weeks earlier than it was last year. That means that barring an unusually large wave of enrollments Friday, the final tally on HealthCare will fall short of last season's total.
On Thursday, Scott Flanders, CEO of the web-based insurance brokerage eHealth, said that at minimum he expected there to be 2 million fewer enrollments this season than last season.