An impromptu appearance in the White House briefing room offered the president an opportunity to trumpet the new figures, which beat initial projections by 1 million. With an eye toward November, Obama castigated Republicans for continuing to seek out every opportunity to thwart the Affordable Care Act.
"This thing is working," Obama said of his signature domestic achievement.
Touting modest progress on another front, Obama said 35 percent of enrollees are under 35 years old, suggesting that in the final weeks of enrollment, the administration managed to sign up higher numbers of younger, healthier people who are critical to the law's viability.
The most coveted age group comprises those between 18 and 34 years old. White House officials said that for the 36 states where the federal government is taking the lead, 28 percent are in that age group - a step in the right direction from March, when the administration said just 25 percent were 18 to 34.
In a sharp rebuke to his political opponents, Obama called out states that have refused to embrace an expansion of Medicaid under "Obamacare," arguing that their opposition was rooted in nothing more than sheer ideology and political spite.
"That's wrong. It should stop," he said. "Those folks should be able to get health insurance like everybody else."
Although the first year's open enrollment season for the exchanges closed on March 31, the administration is still tallying the number of total enrollees. States managing their own exchanges have been slower to report data, and some Americans who started applications before the deadline were given extra time to complete their enrollment.
The demographic figures also give Democrats an opportunity to blunt the pessimism of Republicans, some of whom have accused the White House of "cooking the books" by announcing large overall enrollment numbers that tell only part of the story.
"They still can't bring themselves to admit that the Affordable Care Act is working," Obama said. "The longer we see the law benefiting millions of people, the more we see accusations that the law is hurting people being completely debunked."
Democrats have been hoping that better-than-expected results could help their candidates reclaim the political high ground on "Obamacare" before Election Day. Seven months out, Democrats are seeking to turn the page on the law's disastrous debut in October, when HealthCare.gov was virtually unusable. Obama seemed to affirm that strategy last week when he announced that Health and Human Services Secretary Kathleen Sebelius, who became the face of the rollout failure, was stepping down.
Polling shows the law remains unpopular in much of the country, but Democrats plan to argue that by trying to repeal the law, Republicans are actively working to take health care away from 8 million Americans.
Although the new figures provide some clarity about how well the exchanges performed, there are still plenty of unknowns.
Officials haven't released a tally of how many enrollees were previously uninsured and are thus gaining health care thanks to the law. It's also unclear how many enrollees sealed the deal by paying their first month's premium to the insurance companies.
Republicans seized on those uncertainties to argue that Obama is hyping figures that obscure the real damage the law is inflicting - like higher premiums, smaller provider networks and canceled policies, according to Senate Minority Leader Mitch McConnell, R-Ky.
"It's long past time for Washington Democrats to work with us to remedy the mess they created - and that means repealing this law and replacing it with real reforms that actually lower costs," McConnell said.
As Obama's health law begins to look more viable, Democrats have been seeking to change the political debate from one about repeal to one about fixing lingering issues with the law.
Obama said it's "absolutely possible" to make improvements, but that it would require a change of attitude from Republicans. But election-year posturing and the GOP's reluctance to be seen as embracing "Obamacare" make than an unlikely proposition.
The president's upbeat assessment came shortly after he and top aides had separate meetings with leading insurance executives and state insurance commissioners.
"I think that's a pretty good number in terms of trying to make sure we have a healthy pool," Montana's insurance commissioner, Monica Lindeen, said of the surge in younger enrollees.
In other positive news for Obama's health care law, California's state-run insurance exchange reported Thursday that nearly 1.4 million Californians had enrolled by the end of open enrollment, besting original projections by almost 100,000 people.