The gains were particularly significant for poor Americans in part because a larger share of them lacked health insurance to begin with. But the poor also benefited from the subsidies, and from a vast expansion of Medicaid, the government insurance program for the poor. More than 20 states refused to expand the program, and many experts said the gains would have been even larger had they done so.

While black Americans under the age of 65 made the biggest gains, Hispanics in the same age group also benefited substantially, with the share of uninsured dropping by nearly 17 percent from 2013 to 25.2 percent. The share of whites who were uninsured fell to 9.8 percent, down from 12.1 percent in 2013.

“The law has had a more pronounced effect in covering African-Americans than whites,” said Larry Levitt, a director at the Program for the Study of Health Reform and Private Insurance at the Kaiser Family Foundation, a health research organization. He said part of the reason was that blacks were more likely to be poor, and the law specifically targeted poor Americans for help with coverage. “If all states were expanding Medicaid, you’d see an even bigger effect.”

Many of the states that declined to expand Medicaid were those with the highest share of their populations that were uninsured. They were also home to a large proportion of the country’s poor black residents. Low- and middle-income families above the poverty level are eligible for subsidies to pay for private insurance, but those below are not, and in states that did not expand Medicaid, they were left without coverage.

The share of uninsured among the poor and lower middle class, called “near poor” by the federal government, declined by 7 percentage points and 7.6 percentage points respectively, compared with a 2.5 percentage point decline for Americans who were not poor. The measure was for ages 18 to 64-year-olds.