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President Barack Obama announced Thursday that eight million people have selected a private health insurance plan through the Affordable Care Act.

"This thing is working," he said of the law during an announcement during a statement in the White House briefing room.

Obama knocked Republicans for continuing to support repeal of the law, saying that the GOP is processing the "stages of grief" about its successes, saying the party is "not at acceptance yet."

And he said Democrats running for reelection this fall should "forcefully defend and be proud of the fact that millions of people ... we're helping because of something we did."

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In his remarks, the president also said that 35 percent of people who enrolled through the federal marketplace are under the age of 35.

Twenty eight percent of those who have signed up are between 18 and 34 years old, according to the White House.

The final numbers from the initial enrollment period come after Obama announced on April 1 that 7.1 million Americans had enrolled in the program. Shortly before her resignation last week, Secretary of Health and Human Services Kathleen Sebelius announced an uptick in that number to 7.5 million.

The final number announced Thursday includes the registrations of individuals who were “in line” to sign up but whose applications were not fully processed by the March 31 deadline.

The next open enrollment period for 2015 coverage is November 15, 2014 to February 15, 2015.

In addition to his announcement, Obama met earlier Thursday with state insurance commissioners.

One attendee -- former Sen. Ben Nelson, now the CEO of the National Association of Insurance Commissioners -- told NBC News that the president said about two million people signed up at the end of the enrollment period and that there was "some optimism" that a good percentage of those who enrolled in that final rush are young people.

After a rocky start last fall, the data exceeds original estimates of how many users the law would attract. Democrats call the numbers a sign of the Affordable Care Act’s success and argue that it will become more popular as more Americans start receiving its benefits.

The president’s party also points to new polling that shows a significant drop in the rate of uninsured people in states that have set up a health care exchange and expanded Medicaid coverage, compared to those that have not taken such steps. In a poll released Wednesday, Gallup found that the uninsured rate in the first quarter of 2014 had dropped to 13.6 percent in states that have implemented those measures.

NBC’s Kristen Welker contributed to this report.