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…

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.”…

Dan Pfeiffer, the White House communications director, described the tax power as an alternative source of authority.

“The Commerce Clause supplies sufficient authority for the shared-responsibility requirements in the new health reform law,” Mr. Pfeiffer said. “To the extent that there is any question of additional authority — and we don’t believe there is — it would be available through the General Welfare Clause.”