WASHINGTON—Increasing the national minimum wage to $15 an hour would deliver a raise for millions of U.S. workers but could also cost 1.3 million Americans their jobs, according to a government forecast that complicates congressional Democrats’ push to more than double the federal pay floor.

If the federal minimum wage were raised to $15 an hour in 2025, as House Democrats have proposed, a significant number of Americans would likely lose their jobs, a study released Monday by the nonpartisan Congressional Budget Office found.

But the study also estimates that in an average week in 2025—the year after the House measure would take full effect—a $15-an-hour minimum wage would boost the pay of 17 million workers who would otherwise earn less than $15 per hour. It could also raise the pay of 10 million workers who otherwise would earn slightly more than $15 an hour. Raising the federal minimum to $15 would lift 1.3 million Americans out of poverty, the CBO said.

The agency prepared the report on the effects of lifting the wage from $7.25 an hour ahead of next week’s expected House vote on a bill that would gradually lift the pay floor to $15 an hour by 2024.

“If you look at the whole report, there’s no question there are significant benefits for a massive number of people that far outweigh whatever the cost might be,” said Rep. Bobby Scott (D., Va.), chairman of the House labor committee and one of the bill’s top supporters.