In a brief defending the law, the Justice Department says the requirement for people to carry insurance or pay the penalty is “a valid exercise” of Congress’s power to impose taxes.Congress can use its taxing power “even for purposes that would exceed its powers under other provisions” of the Constitution, the department said. For more than a century, it added, the Supreme Court has held that Congress can tax activities that it could not reach by using its power to regulate commerce.

While Congress was working on the health care legislation, Mr. Obama refused to accept the argument that a mandate to buy insurance, enforced by financial penalties, was equivalent to a tax.

“For us to say that you’ve got to take a responsibility to get health insurance is absolutely not a tax increase,” the president said last September, in a spirited exchange with George Stephanopoulos on the ABC News program “This Week.”

When Mr. Stephanopoulos said the penalty appeared to fit the dictionary definition of a tax, Mr. Obama replied, “I absolutely reject that notion.”