The federal Obamacare exchange had ts biggest week so far this season — and could break that record this week.
About 1.3 million people signed up for health insurance plans between Dec. 6 and last Saturday, officials said Wednesday.
The surge of customers, which came as a key deadline for enrollment approached, brings HealthCare.gov's sign-up tally to 4.17 million people since enrollment began for 2016 plans on Nov. 1. The federally run marketplace serves residents in 38 states.
Traffic on HealthCare.gov and Obamacare exchanges run by individual states has been very heavy this week, prompting Obamacare officials in most of the United States to give consumers more time to sign up for plans that take effect Jan. 1.
Tuesday was supposed to be the deadline for coverage effective by New Year's Day.
But because of the large volume of shoppers, which clogged enrollment web sites and call centers, HealthCare.gov, as well as the exchanges run by California, New York, Minnesota and the District of Columbia, have all extended that deadline.
California's Obamacare marketplace, which is the largest state-run exchange in terms of customers, said that more than 197,000 people had signed up for coverage in 2016 as of Tuesday night. That tally includes more than 54,000 people who signed up on Monday and Tuesday alone.
HealthCare.gov, Covered California and the DC Health Link now have deadlines of Thursday night; New York State of Health's deadline is Saturday night, and Minnesota's MNsure exchange has a Dec. 28 deadline.
The broader open-enrollment deadline for having coverage for 2016 is Jan. 31. After that, people who don't have some kind of health coverage will be subject to an Obamacare fine of the higher of $695 or 2.5 percent of household income, unless they qualify for an exemption.
"The unprecedented demand over the last several days continues to show that coverage through HealthCare.gov is something millions of Americans want and need," said U.S. Health and Human Services Secretary Sylvia Burwell.
Officials said "hundreds of thousands" of people selected plans on Monday and Tuesday on the federal exchange. And approximately 1 million consumers have left their contact information at HealthCare.gov to hold their place in line. People in line will be able to get coverage effective Jan. 1.
"We urge those who left their names with the marketplace to come back to Healthcare.gov or the call center and complete their application for coverage starting Jan. 1," Burwell said.
About 36 percent of the sign-ups seen on HealthCare.gov so far this season — or about 1.5 million people — are new customers, official said.
The states with the highest number of sign-ups so far are Florida, with almost 835,000, Texas, which has more than 474,600, and North Carolina, where more than 280,000 people signed up.
For enrollment to be official, people must make their first month's premium payments.
Burwell has said she expects about 10 million people to be enrolled in Obamacare plans sold by HealthCare.gov and the other state-run exchanges by the end of 2016.
HealthCare.gov and the state-run marketplaces are the only locations where buyers of individual insurance plans can receive federal subsidies that can reduce the price of their monthly premiums. More than 80 percent of exchange customers qualify for such financial aid as a result of having low or moderate incomes.