WASHINGTON — Eight million people have signed up for health care through new insurance exchanges, President Obama said Thursday, besting expectations and offering new hope to Democrats who are defending the law ahead of the midterm elections.

Obama appeared 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.

Also on Thursday, Obama and Vice President Joe Biden met privately with a group of state health officials, including several representing the nonprofit health co-operatives.

Kevin Lewis, CEO of Maine Community Health Options, was part of the contingent that met with Obama and Biden for more than an hour.

Lewis said he talked about how Maine’s grass-roots efforts resulted in signing up more people in Maine than the national average. The Maine co-op has captured about 80 percent of the insurance exchange market in the state has been highlighted in the national media as an Affordable Care Act success story.

“It was really a meeting of celebration, of reaching the 8 million,” Lewis said. “But it was also about looking ahead, that the work did not end when the enrollment period ended.”

Democrats have been anxiously awaiting the age figures, 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 accused the White House of “cooking the books” by announcing large overall enrollment numbers before releasing more detailed figures.

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. 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.

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.

Staff Writer Joe Lawlor contributed to this story.