Obamacare's final enrollment tally topped 8 million people by Tuesday's sign-up deadline, President Barack Obama announced Thursday. Obama was clearly pleased about hitting that benchmark — which exceeded by 1 million the estimate federal officials made last summer, before the launch of government exchanges selling Affordable Care Act insurance plans. At the same time, the White House revealed Thursday that final enrollments by young adults over the six-month sign-up period was lower than had been hoped for by ACA advocates. Just 28 percent of all sign-ups came from people between the ages of 18 and 34 — well below the 40 percent target that some insurance experts have said is needed to keep premium rates stable in the future. Still, Obama said, "This thing is working," as he urged Republicans to drop their repeated efforts to repeal or substantially weaken the Affordable Care Act through votes in the House of Representatives. "They were wrong about trying to repeal a law that is working, when they have no alternative."

President Barack Obama delivers remarks from the Brady Press Room at the White House in Washington, DC, April 17, 2014. Jim Watson | AFP | Getty Images

"I'm still puzzled why they have made this their sole agenda item when it comes to politics."

"I think we can all agree that it's well past time to move on," Obama said. In response, the National Republican Congressional Committee bluntly tweeted, "No, we can't." Obama said the ACA, while not perfect, has greatly expanded health coverage to people who previously were unable to get it due to pre-existing conditions. And, he said, the law has helped to slow the growth of health-care costs since it was enacted in 2010.

This is the first year that exchange-sold plans are in effect. It is also the first year in which nearly all Americans must have some form of health insurance or face a tax penalty of up to 1 percent of their taxable income next year.

The announced tally means that as many as 900,000 people completed their enrollments in the past two weeks, taking advantage of a extension of the original March 31 open-enrollment sign-up deadline until this past Tuesday. That extension was available to most people who had started applications on Obamacare exchanges by March 31, but had trouble finishing their enrollments by the cutoff date for technical or other reasons. The White House pointed out that 35 percent of total enrollment came from people under the age of 35. That group includes children whose parents enrolled in coverage for their families. Read MoreCalifornia's Obamacare deadline sign-up surge



Source: Healthcare.gov

The White House also noted that another 5 million people enrolled in individual insurance plans that are compliant with Affordable Care Act rules outside of the government-run exchanges, which include HealthCare.gov.

And another 3 million additional people have enrolled in government-run Medicaid or CHIP health coverage as of February. Many of those Medicaid enrollments were in states that have expanded their eligibility requirements for that program to encompass most poor adults. Twenty-four states have not expanded Medicaid coverage despite the fact that the federal government is paying 100 percent of the costs of covering the newly eligible for the first three years. Obama said that "does frustrate me." He noted that those states are leaving about 5 million people without health coverage that they could otherwise get at no cost, and argued they have "chosen not to expand Medicaid for no other reason other than political spite." "They have chosen not to provide health insurance to their citizens," Obama said. "That's wrong, that should stop, those folks should be able to get health insurance just like other folks." The enrollment tallies for private Obamacare plans announced in recent weeks have surprised even supporters of the Affordable Care Act.

The first two months of enrollment after the Oct. 1 launch were a disaster, as many people who tried to sign up were unable to do so because of technical problems on federally run HealthCare.gov and several state-run exchanges. In the first month of enrollment, just 106,185 people enrolled in private plans, much less than the 494,620 that the Congressional Budget Office had originally predicted would sign-up during that period.

November was barely better, with only 258,497 people signing up that month. That was also well below the 706,600 originally projected by the CBO for the month. Read MoreObamacare helped 10 million uninsured, says Gallup

