The Obama administration said on Monday that 16.4 million uninsured people had gained health coverage since major provisions of the Affordable Care Act began to take effect in 2010, driving the largest reduction in the number of uninsured in about 40 years.
Since the first open enrollment period began in October 2013, the officials said, the proportion of adults lacking insurance has dropped to 13.2 percent, from 20.3 percent.
Sylvia Mathews Burwell, the secretary of health and human services, said the data revealed “the largest reduction in the uninsured in four decades.” Many people gained coverage after the creation of Medicare and Medicaid in 1965.
White House officials said the figures disproved the charges leveled by some Republicans opposed to the law, including governors who have declined to expand their Medicaid programs.
“We’ve seen tens of millions, if not hundreds of millions, of dollars being spent by the president’s political opponents to distort the facts about the true impact of the Affordable Care Act,” Josh Earnest, the White House press secretary, said on Monday. “We’re very pleased about the impact that this has had in expanding coverage for more Americans.”
Polls show that public opinion on the health care law is still deeply divided, and its future could be an issue in next year’s elections. Richard G. Frank, an assistant secretary of health and human services, said the new data showed that “the Affordable Care Act is working.”
Officials said they were publishing the figures to demonstrate its progress on the fifth anniversary of the law, which President Obama signed on March 23, 2010.
Since October 2013, Mr. Frank said, 14.1 million uninsured people ages 18 to 64 have gained insurance. In addition, he said, 2.3 million young adults were covered from 2010 to October 2013 because they were allowed to remain on their parents’ health plans until the age of 26 under a provision of the law.
Federal officials said they did not know how many of the 14.1 million newly insured Americans had gained private coverage through health insurance exchanges and how many had obtained their coverage through Medicaid. Many states say their Medicaid enrollment numbers are larger.
Senator John Barrasso of Wyoming, the chairman of the Senate Republican Policy Committee, said: “Millions of people have lost coverage they liked, and out-of-pocket costs continue to rise. Coverage does not equal care.”
Moreover, he said, the addition of people to the Medicaid rolls is “hardly worth celebrating.”
Mr. Frank said the estimates were obtained by applying a statistical model to data from a large private survey conducted for the Gallup-Healthways Well-Being Index. He predicted that the estimates would be confirmed in surveys conducted by the Census Bureau and the Centers for Disease Control and Prevention.
The administration’s estimates were similar to those of the Congressional Budget Office. In a report last week, the budget office said that the health care law had reduced the number of uninsured people under the age of 65 by 17 million, to a total of 35 million people.
Joseph R. Antos, an economist at the American Enterprise Institute, a right-of-center think tank, said it was not possible to say how much of the reduction in the uninsured resulted from improvements in the economy and how much was attributable to the Affordable Care Act.
All the changes were measured against levels of insurance coverage in the 21-month period from January 2012 to September 2013, just before the beginning of open enrollment under the health law.
For decades, the proportion of blacks who are uninsured has been higher than the proportion for the nation as a whole. If the Obama administration is correct, that gap may be disappearing.
The administration estimated that 2.3 million black adults had gained coverage since October 2013, lowering the proportion who are uninsured to 13.2 percent, the same as the average for all adults.
Federal officials said that 6.6 million non-Hispanic white adults and 4.2 million Latino adults had also gained coverage. The proportion of Latinos who were uninsured dropped to 29.5 percent, from 41.8 percent, the administration said, and the proportion for whites declined to 9 percent, from 14.3 percent.