Health and Human Services Secretary Kathleen Sebelius testifies on Capitol Hill in Washington, Thursday, April 10, 2014, before the Senate Finance Committee hearing on the HHS Department's fiscal Year 2015 budget. Sebelius said 7.5 million Americans have now signed up for health coverage under President Barack Obama's health care law. That's a 400,000 increase from the 7.1 million that Obama announced last week at the end of the law's open enrollment period. The figure exceeded expectations, a surprise election-year success for the law after a disastrous roll-out. (AP Photo/Susan Walsh)

Enrollment for the president's health care law has grown to 7.5 million Americans, the Obama administration announced Thursday, handing the president and his Democrats better numbers to tout in the face of election-year attacks.

Sign-ups for the law, which remains unpopular with many Americans, stood at 7.1 million last week, past a target number that had once seemed unattainable

But people who had already started signing up when the enrollment period closed March 31, or who had trouble signing up, were given extra time to finalize their applications. Four hundred thousand more have now done so, Health and Human Services Secretary Kathleen Sebelius said in testimony before the Senate Finance Committee. Sebelius said she expects the figure to continue to grow.

The original number already exceeded expectations, a surprise success after a disastrous roll-out and welcome news for Democrats who've been forced to defend their support for the law derided by critics as "Obamacare." Democratic senators took turns Thursday praising the robust sign-up numbers.

"We now know millions of people who have directly benefited from the Affordable Care Act," said Democratic Sen. Ben Cardin, while bemoaning the continuing attempts by opposition Republicans to repeal the law.

Health and Human Services Secretary Kathleen Sebelius listens on Capitol Hill in Washington, Thursday, April 10, 2014, during the Senate Finance Committee hearing on the HHS Department's fiscal Year 2015 budget. Sebelius said 7.5 million Americans have now signed up for health coverage under President Barack Obama's health care law. That's a 400,000 increase from the 7.1 million that Obama announced last week at the end of the law's open enrollment period. The figure exceeded expectations, a surprise election-year success for the law after a disastrous roll-out. (AP Photo/Susan Walsh)

About 50 million Americans lacked as the law began taking effect, and its passage in 2010 has been the No. 1 legislative achievement of Barack Obama's presidency.

The new figure seemed unlikely to tamp down Republican attacks over the law and its new requirement for Americans to carry insurance or face penalties. The Republican Party sees discontent over the law as a winning political issue heading into November midterm elections where Republicans are aiming to defend their control of the House, and perhaps retake the Senate.

The administration has not said how many of those who already have signed up for insurance closed the deal by paying their first month's premiums. Also unknown is how many were previously uninsured—the real test of Obama's health care overhaul. In addition, the law expands coverage for low-income people through Medicaid, the government program for the poor, but only about half the states have agreed to implement that option.