WASHINGTON — The Trump administration broadened its attack on the Affordable Care Act on Monday, telling a federal appeals court that it now believed the entire law should be invalidated.

The administration had previously said that the law’s protections for people with pre-existing conditions should be struck down, but that the rest of the law, including the expansion of Medicaid, should survive.

If the appeals court accepts the Trump administration’s new arguments, millions of people could lose health insurance, including those who gained coverage through the expansion of Medicaid and those who have private coverage subsidized by the federal government.

“The Justice Department is no longer asking for partial invalidation of the Affordable Care Act, but says the whole law should be struck down,” Abbe R. Gluck, a law professor at Yale who has closely followed the litigation, said on Monday. “Not just some of the insurance provisions, but all of it, including the Medicaid expansion and hundreds of other reforms. That’s a total bombshell, which could have dire consequences for millions of people.”