President Biden warned on Tuesday that if former President Donald J. Trump returned to office by winning next month’s election, he would enact policies that could deprive tens of millions of Americans of health insurance coverage and explode the price of prescription drugs.

During a speech in Concord, N.H., Mr. Biden assailed Mr. Trump for repeatedly trying to repeal the 2010 Affordable Care Act, and he mocked the former president for offering only ephemeral and unspecified “concepts of a plan” to replace it.

“My predecessor, the distinguished former president, he wants to replace the Affordable Care Act with what he calls the ‘concept of a plan,’” Mr. Biden told an audience at NHTI — Concord’s Community College. “I’ve heard that ‘concept of a plan’ now for almost eight years. Concept of a plan. What the hell is a concept of a — he has no concept of anything! No plan!”

Mr. Biden continued. “If we don’t elect Kamala and he gets elected,” he said, referring to Vice President Kamala Harris, “Trump could kick up to 45 million people off of health insurance. Forty-five million! Over 100 million people could lose health care coverage because they have a preexisting condition. The only reason they can get it is because of the Affordable Care Act.”

About 21 million people signed up for a health plan this year through the marketplaces set up by the Affordable Care Act. All told, nearly 50 million Americans have been covered by such plans since they were first created a decade ago. A federal government analysis released last month found that 50 million to 129 million non-elderly Americans have some sort of preexisting condition, which could put them at risk of losing coverage or being charged exorbitant prices without the protection of the 2010 law.

Democrats hope to make health care a defining issue in the final two weeks of the campaign, seizing on Mr. Trump’s comment at his debate last month with Ms. Harris that he would still try to eliminate the Affordable Care Act if he could come up with a better version. Pressed on whether he actually had such a plan, he said, “I have concepts of a plan” and that “you’ll be hearing about it in the not-too-distant future.”

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