Maria Elena Santa Coloma, right, an insurance adviser with UniVista Insurance company, helps Shessy Gonzalez sign up for a health plan under the Affordable Care Act, on Dec. 15, 2015, in Miami. (Joe Raedle/Getty Images)

About 4 million Americans gained health insurance last year, decreasing the nation’s uninsured rate to 9.1 percent, the lowest level since before the Great Recession, according to new federal figures.

The figures, released Tuesday from a large annual Census Bureau survey, show that the gains were driven primarily by an expansion of coverage among people buying individual policies, rather than getting health benefits through a job. This includes, but is not limited to, the kind of coverage sold on the insurance exchanges that began in 2014 under the Affordable Care Act.

The decrease in the share of people who were uninsured slowed from the year before. Between 2013 and 2014, some 9 million people gained coverage — more than twice the increase as from 2014 to last year. Some 29 million people were uninsured in 2015

Taken together, the Census Bureau findings provide evidence that “the ACA is continuing to decrease” the ranks of the uninsured, said Chris Sloan, a senior manager at Avalere, a health-care consulting firm. “It’s not as fast as 2014. Nobody expected it to be as fast” as during the first year that the law created the new insurance marketplaces and expanded Medicaid in states that chose to do so.

Over the two-year period from 2013 to 2015, the figures show, the nation’s Medicaid rolls expanded by two percentage points, while the share of Americans buying private coverage on their own swelled by nearly five percentage points. In contrast, employer-based insurance — still the dominant form of health coverage in the United States — did not change significantly, the figures show.

Around the country, improvements in insurance coverage last year were greater in the 31 states, plus the District of Columbia, that have expanded their Medicaid programs, compared with those that have not. Originally, the health-care law envisioned a nationwide expansion of Medicaid to include people with slightly higher incomes than were eligible before, but a 2012 Supreme Court decision gave each state the latitude to decide whether to participate.

Health insurance improves consumers’ ability to get medical care when they need it, and the proportion of people without insurance is influenced by a variety of factors, including economic trends. But during the past few years, the uninsured rate has come to be regarded, by proponents and detractors alike, as a measuring stick for how well the controversial health-care law, enacted in 2010 to revise the U.S. health care system, is working. One of the law’s main intentions has been to make affordable health coverage more available.

Within an hour of the Census Bureau figures’ release, a trio of White House economic advisers issued a joint statement that said: “Every State has seen declines in its uninsured rate since 2013 as the major coverage provisions of the Affordable Care Act have taken effect.”

Six years after its creation, the law still is a defining partisan issue, and it has filtered into this year’s presidential politics. Like many Republicans, GOP nominee Donald Trump says he would repeal the law. He would like to remove the requirement that most Americans carry health insurance and convert Medicaid into state block grants.

Democratic nominee Hillary Clinton says she would “defend and expand” the law. She says she would, for instance, try to constrain consumers’ out-of-pocket costs for health care and prescription drugs, and create a public insurance alternative in every state to compete with private insurers in the ACA marketplaces.

From 2014 to last year, the uninsured rate decreased by 1.3 percentage points — compared with a drop of nearly three percentage points the year before.

The slower pace of improvement is line with the trajectory of enrollment in the ACA marketplaces, which has been growing more slowly after 8 million people signed up for the first year. For 2015, nearly 13 million signed up, but, as happens every year, some dropped or lost their insurance as the year progressed. The Obama administration predicted slight enrollment growth for the current year.

The Census Bureau’s Current Population Survey is widely considered the most reliable data for gauging how the nation is faring with the insurance coverage. Its findings are in sync with other recent evidence. Last week, the Centers for Disease Control and Prevention reported that, for the first three months of this year, the uninsured rate was 8.6 percent — a record low. The CDC report relies on the National Health Interview Survey, which asked respondents whether they are uninsured at the time, rather than for an entire year, as the Census Bureau measures.

The new figures also show that the country is making some headway with parts of the population that traditionally have had the least access to health coverage, though racial and ethnic disparities persist. Hispanics still are least likely to have insurance, with 16.2 percent uninsured, though they made the biggest gains last year. There was no improvement among blacks, 11.1 percent of whom lacked coverage. In comparison, 6.7 percent of whites were uninsured last year.