The scramble to sign up under Obama's health care law at the end of a six-month enrolment window caused website glitches and long lines at on-the-spot enrollment centers.
But Republicans renewed a vow to repeal the law, which they say costs jobs, handcuffs small businesses and represents a government power grab in the private health care market.
More than two million people visited the Healthcare.gov website on Monday and another 840,000 called telephone enrolment centres, a White House official said, terming the latest figures unprecedented.
It remained unclear, however, how many of those people went through the whole process of securing Affordable Care Act insurance plans.
"I signed the #ACA so millions could know the peace of mind that comes with health insurance," Obama wrote in a tweet.
In a rare moment of triumph in Obama's so far grim second term, senior White House officials saw the deadline day rush as vindication after the disastrous rollout of the health care website late last year.
"There has been a remarkable story since the dark days of October and November," said White House spokesman Jay Carney.
Officials said a problem with the Healthcare.gov website stopped people from establishing new accounts several times.
But Health and Human Services Secretary Kathleen Sebelius nevertheless hailed the "huge surge day" in an interview with HuffPost Live.
It will be months, however, before the long-term success of Obamacare in broadening access and making medical insurance affordable can be assessed, and the law remains a political albatross for the president and fellow Democrats.
The law demands that all Americans have health insurance or pay a fine, but offers subsidies for the less well-off to sign up.
The White House said that the final enrolment figures for the first year would be substantially higher than the six million figure already recorded. Officials had earlier set seven million enrolments as the standard for success.
The health care law is Obama's signature domestic political achievement and the most sweeping social reform in decades in the United States.
Yet its passage and implementation ignited a partisan battle and highlighted deep ideological divisions cleaving the United States.
'Cooking the books'
Republicans, however, are rubbishing administration claims, warning that the new law is crippling small business owners and costing jobs.
"I think they're cooking the books," Republican Senator John Barrasso told "Fox News Sunday."
Republican House Speaker John Boehner warned "the problem was never just about the website it's the whole law."
"Millions of Americans are seeing their premiums rise, not the lower prices the president promised," he added, renewing his vow to repeal the law.
Objective judgments on Obamacare are hindered by a lack of detail in enrolment figures offered by the administration.
It is unknown how many young people have signed up an important indicator since they are needed to subsidize higher costs of older, sicker patients.
If Obamacare pools skew too old, the cost of premiums could rise next year.
Also unclear is whether everyone who registered for a health plan actually paid for it.
Another unknown is what percentage of those signing up had no prior coverage a key metric in assessing if the law helped 40 million Americans who lack insurance.
Vice President Joe Biden visited a "one stop" sign-up centre in Washington, and told people waiting to enroll that although the process was complicated, it was worth it.
"You are going to be better off for it. The country is going to be better off for it," Biden said.
As a whole, the health care law remains unpopular: a recent Pew Research poll found that 41 percent approved of it and 53 percent opposed it.
The partisan breakdown of the poll tells the political story - eight percent of Republicans supported the law, compared to 72 percent of Democrats.
The health care row has helped send Obama's approval ratings to record lows and Democrats battling to cling on to Senate seats in conservative states are struggling to shrug off the backlash.