Obama made a surprise appearance in the White House briefing room to trumpet the new figures, which beat initial projections by 1 million people. Equally critical: About 35 percent of those who signed up are under the age of 35, Obama said. Enrolling substantial numbers of younger, healthier Americans is crucial for the law's success.
"This thing is working," Obama said of the Affordable Care Act, his signature domestic achievement.
Obama and Democrats have been anxiously awaiting the age figures, and especially those regarding young people - the most coveted demographic. Younger enrollees tend to be a healthier group overall, so their premiums can help offset higher cost of care for older enrollees. Too few young people in the mix, and the insurance pool could become lopsided and premiums could surge.
The demographic figures also give Democrats an opportunity to blunt the pessimism of Republicans, some of whom have pointedly accused the White House of "cooking the books" by announcing large overall enrollment numbers before releasing more detailed figures that provide a fuller picture.
Following the disastrous rollout of the exchanges in October, when HealthCare.gov was virtually unusable, Democrats have been hoping that higher-than-expected results could help their candidates reclaim the political high ground ahead of the midterm elections. Seven months out from Election Day, Democrats are seeking to turn the page on the law's flawed debut - a strategy underscored last week when Obama 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 use the high enrollment figures to argue that by trying to repeal the law, Republicans are actively working to take health care away from millions of Americans who now rely on the exchanges.
"The repeal debate is and should be over," Obama said.