President Barack Obama on Thursday announced that 8 million people have enrolled in private health-insurance plans under the Affordable Care Act, using a press conference to largely attack Republicans who he said had been proven "wrong" on the issue.

Obama said 35% of the sign-ups were under the age of 35, and that 28% of those who bought policies were in the crucial 18-to-34 age group, a lower number than insurers had hoped for but better than earlier reports. Obama also said premiums are anticipated to be 15% lower than expected.

He criticized Republicans who he said "can’t bring themselves to admit that the Affordable Care Act is working," arguing that the GOP push to repeal Obamacare had come at the expense of productive conversations about creating jobs, improving the economy, and raising the minimum wage.

"The point is, the repeal debate is and should be over. The Affordable Care Act is working," Obama said in the White House briefing room.

The final numbers exceeded the wildest early expectations. After a disastrous rollout of the law, projections were revised downward. Even at the end of February, it seemed unlikely the Obama administration would even hit the revised target of 6 million. By then, only 4.2 million people had signed up for insurance through the exchanges, and momentum appeared to be sagging.

When he opened the press conference to questions from reporters, Obama was asked whether flaws in the health-care law could be improved. He said they could, but that it would be more difficult if Republicans maintained their opposition to Obamacare.

"It is absolutely possible, but it will require a change in attitude on the part of the Republicans," he said.

Obama went on to denounce "certain" Republicans who believe making the Affordable Care Act "better" is "a concession to me." In that environment, Obama said it would be difficult to improve the law.

"I recognize that their party is going through the stages of grief — anger, denial, all that stuff," Obama said of Republicans. "We're not at acceptance yet."

He went on to predict that the GOP wouldn't change its tune until after the midterm elections in November.

Brendan Buck, a spokesman for House Speaker John Boehner, blasted the White House for not releasing a full breakdown of the data, including the number of enrollees who have paid their first month's premium.

"The White House continues to obscure the full impact of Obamacare. Beyond refusing to disclose the number of people who’ve actually enrolled by paying premiums, the president ignores the havoc that this law has wreaked on private plans that people already had and liked," Buck said in a statement.

"Surveys have consistently shown that the overwhelming majority of those who signed up already had insurance. Had this law not led to millions of Americans receiving cancelation notices, many would not have had to sign up for this government-run program. What America really needs is a health care system that is more affordable, more accessible, and of the highest quality, and that’s what House Republicans are working toward."

This post was updated at 4:31 p.m. ET.