WASHINGTON — Nearly 50 million Americans have been covered by health insurance plans through the Affordable Care Act’s marketplaces since they opened a decade ago, according to tax data analyzed by the Treasury Department and published Tuesday.

Federal officials said that the findings represent roughly 1 in 7 U.S. residents, a broad swath of the population that underscores the vast, and seemingly irreversible, reach of the 2010 law.

The timing of the announcement was significant, coming just hours before the presidential debate in Philadelphia, where Vice President Kamala Harris would likely use the Affordable Care Act in her pitch to voters.

Since President Joe Biden took office, more than 18 million Americans have enrolled in marketplace plans for the first time, federal officials said.

In a record, roughly 21 million people signed up for one this year, a trend that health policy experts have largely attributed to major subsidies that have lowered premiums for many Americans purchasing a plan.

Those subsidies, which cost the federal government tens of billions of dollars, are set to expire next year, likely setting off intense congressional negotiations over whether to extend them.

Enrollment has been particularly high in states that have yet to expand their Medicaid programs to cover more adults, an option introduced by the Affordable Care Act that a vast majority of states have taken up. Florida, Georgia and Texas — three large states that have yet to expand Medicaid — had significant numbers of residents flock to the marketplaces in the past decade.