A total of 1,478,250 people signed up on HealthCare.gov since Nov. 1, the start of open enrollment in individual health insurance plans, according to the federal Centers for Medicare and Medicaid Services.
Last year, 1,008,218 people signed up in the first two weeks.
CMS also reported that the number of people applying for coverage and visiting HealthCare.gov was sharply higher than in the same period last year.
The brisk pace of sign-ups and customer interest comes despite fears by Obamacare advocates that enrollment would significantly decrease because of hostile actions by the Trump administration.
Among other things, the administration has slashed budgets for Affordable Care Act outreach and advertising and ordered federal health officials not to participate in state events that encourage enrollment. Advocates also are concerned that sign-ups will be tamped down by the fact that the enrollment season on the federal exchange is only about half the length of last season.
The new enrollment tally comes a day after Senate Republican leaders said they will seek repeal of Obamacare's individual mandate, which requires most Americans to have some form of health insurance coverage during the year or pay a tax penalty.
A leading Obamacare advocacy group, Get America Covered, on Wednesday said that the average daily rate of enrollments on HealthCare.gov is 60 percent higher than it was last year. That statistic takes into account the fact that the 11 day reporting period by CMS this year is one day shorter than the enrollment snapshop released for the first two weeks last year.
This year, an average of 134,386 people per day are signing up, compared to 84,018 people per day last year, according to Get America Covered, which is run by two former Obama administration health officials.
The group said that the widespread availability of "zero dollar" Obamacare plans is likely boosting enrollment so far this year. Most people who buy coverage on an Obamacare exchange qualify for federal subsidies that reduce their premiums, and more people than ever are able to buy a plan that will personally cost them nothing in premiums this season because of that financial assistance.
The enrollment snapshot released by CMS on Wednesday details sign-ups on HealthCare.gov, the federal exchange that sells individidual plans in the 39 states that do not operate their own Obamacare marketplaces.
Not all state-based marketplaces have released their own tallies.
While the Covered California has not provided further updates, a leading seller of Obamacare coverage in the Los Angeles area, LA Care Health Plan, told CNBC on Wednesday its enrollment this sign-up season is 40 percent higher than last season. About 7,000 of those customers are new enrollees, with roughly 3,500 existing customers, according to the company.
Maryland said its enrollment pace was double last year's in the first week; Washington state reported 53 percent higher enrollment in the first eight days; and Rhode Island saw five times the number of enrollees in the first week from last year.
CMS said 876,788 people selected an insurance plan on HealthCare.gov from Nov. 5 through last Saturday.
Of those people, 208,397 were new customers, and 668,391 were returning customers.
Cumulatively, 345,719 new customers have selected plans on HealthCare.gov since enrollment began Nov. 1.
And 1,232,531 customers have renewed coverage on that exchange.
The number of consumers on applications for coverage submitted to HealthCare.gov was 2.92 million in the first two weeks, or about 42 percent higher than the number of consumers on applications during the first two weeks of last year's enrollment period.
The number of unique users on the federal website was 5.8 million in the first two weeks of this season, or 28 percent higher than the number of users last year.
Open enrollment on HealthCare.gov ends Dec. 15. The sign-up season is longer, and ends on various days, on most state-run exchanges.
People who do not have insurance coverage during 2018 are subject to a tax penalty that is the higher of $695 per adult or 2.5 percent of household income.
However, many uninsured people can avoid the penalty if they qualify for a wide range of exemptions.