The drop in insurance coverage in 2018 is relatively small compared with the long-term trend, but it suggests that policy changes under the Trump administration, which has been hostile to the health law, have made a difference.

The administration cut back on advertising and enrollment assistance, programs that helped low-income people learn about the new insurance programs, among other changes that may have depressed the number of people signing up for health plans. The government also announced that it might begin counting Medicaid enrollment as a strike against immigrants who are seeking green cards or citizenship — a policy that became final this year. Insurance coverage for Americans of Hispanic origin fell last year, according to the report.

The administration’s decision in 2017 to eliminate a subsidy program contributed to large price increases for health insurance in the Obamacare marketplaces in many parts of the country the next year. Research from the Department of Health and Human Services shows that more than a million Americans who were previously buying their own insurance left the market in 2018.

But the Census Bureau figures show that the main change in the uninsured rate came from declines in Medicaid coverage. Urged by the administration, which expressed concerns about the program’s integrity, several states started asking families to prove their eligibility for Medicaid more often in 2018. The number of Americans covered by Medicaid and the Children’s Health Insurance Program fell by more than 1.6 million last year, according to administrative data.

“If you increase red tape you are going to lose people, many of whom are actually eligible for the coverage,” said Joan Alker, a research professor at Georgetown University. She said she was particularly disheartened to see declines in the number of children with health insurance.

But Brian Blase, a former special assistant to Mr. Trump for health care policy, pointed to a recent study suggesting that some Americans who had enrolled in Medicaid in the early years of Obamacare were not eligible for it.

“My sense is in 2018 states probably started tightening eligibility,” said Mr. Blase, who is now president of the consulting and research firm Blase Policy Strategies.