Administration officials say the tax argument is a linchpin of their legal case in defense of the health care overhaul and its individual mandate, now being challenged in court by more than 20 states and several private organizations.

Under the legislation signed by President Obama in March, most Americans will have to maintain “minimum essential coverage” starting in 2014. Many people will be eligible for federal subsidies to help them pay premiums.

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.”

Congress anticipated a constitutional challenge to the individual mandate. Accordingly, the law includes 10 detailed findings meant to show that the mandate regulates commercial activity important to the nation’s economy. Nowhere does Congress cite its taxing power as a source of authority.

Under the Constitution, Congress can exercise its taxing power to provide for the “general welfare.” It is for Congress, not courts, to decide which taxes are “conducive to the general welfare,” the Supreme Court said 73 years ago in upholding the Social Security Act.

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.”

The law describes the levy on the uninsured as a “penalty” rather than a tax. The Justice Department brushes aside the distinction, saying “the statutory label” does not matter. The constitutionality of a tax law depends on “its practical operation,” not the precise form of words used to describe it, the department says, citing a long line of Supreme Court cases.

Moreover, the department says the penalty is a tax because it will raise substantial revenue: $4 billion a year by 2017, according to the Congressional Budget Office.

In addition, the department notes, the penalty is imposed and collected under the Internal Revenue Code, and people must report it on their tax returns “as an addition to income tax liability.”

Because the penalty is a tax, the department says, no one can challenge it in court before paying it and seeking a refund.

Jack M. Balkin, a professor at Yale Law School who supports the new law, said, “The tax argument is the strongest argument for upholding” the individual-coverage requirement.

Mr. Obama “has not been honest with the American people about the nature of this bill,” Mr. Balkin said last month at a meeting of the American Constitution Society, a progressive legal organization. “This bill is a tax. Because it’s a tax, it’s completely constitutional.”

Mr. Balkin and other law professors pressed that argument in a friend-of-the-court brief filed in one of the pending cases.

Opponents contend that the “minimum coverage provision” is unconstitutional because it exceeds Congress’s power to regulate commerce.

“This is the first time that Congress has ever ordered Americans to use their own money to purchase a particular good or service,” said Senator Orrin G. Hatch, Republican of Utah.

In their lawsuit, Florida and other states say: “Congress is attempting to regulate and penalize Americans for choosing not to engage in economic activity. If Congress can do this much, there will be virtually no sphere of private decision-making beyond the reach of federal power.”

In reply, the administration and its allies say that a person who goes without insurance is simply choosing to pay for health care out of pocket at a later date. In the aggregate, they say, these decisions have a substantial effect on the interstate market for health care and health insurance.

In its legal briefs, the Obama administration points to a famous New Deal case, Wickard v. Filburn, in which the Supreme Court upheld a penalty imposed on an Ohio farmer who had grown a small amount of wheat, in excess of his production quota, purely for his own use.

The wheat grown by Roscoe Filburn “may be trivial by itself,” the court said, but when combined with the output of other small farmers, it significantly affected interstate commerce and could therefore be regulated by the government as part of a broad scheme regulating interstate commerce.