WHILE POLICY considerations will infuse the Supreme Court arguments Tuesday about the health-care mandate, the session will focus on this legal question: Does the Constitution give Congress the power to order all individuals above a certain income level to buy health insurance? This is the topic of the Supreme Court’s second of three days of consideration of the health-care reform act.

Twenty-six states, the National Federation of Independent Business and several individuals argue that Congress has overreached. To them, the mandate represents a top-down, big-government imperative that threatens liberty. They acknowledge that the Constitution gives Congress robust powers to regulate interstate commerce and the individuals and companies involved in such commerce. But they argue that Congress cannot force individuals to make a purchase from a private company in the marketplace.

“[I]f Congress has the power not just to regulate commercial suppliers and those who voluntarily enter the market, but to compel demand as well, then we have truly entered a brave new world,” the states argue in a court filing. And if the Supreme Court blesses such a move, they say, there will be nothing to stop Congress from mandating purchases of everything from automobiles to vegetables. The mandate, they conclude, is “as unbounded as it is unprecedented.”

These are serious arguments. But we believe the government has the better of the policy and legal case for why the individual mandate is necessary and constitutional.

Health-care services account for some 17 percent of the country’s gross domestic product; today, the average family pays an additional $1,000 annually in the form of higher premiums to subsidize the costs incurred by those who receive care but do not carry insurance. The mandate is an indispensable tool for achieving the government’s compelling goals of universal coverage and lower costs. Insurance companies would be unable to offer affordable coverage to those with preexisting conditions, for example, unless they also were guaranteed enrollment of the young and healthy customers who are less likely to consume health-care services.

In the recent past, the Supreme Court has struck down attempts by Congress to use the Constitution’s Commerce Clause to promulgate laws that had no connection to commercial activity, including those involving guns near schools and violence against women. Yet it has upheld Congress’s Commerce Clause power to reach individuals who were not obviously involved in commercial activity — most famously, the Depression-era farmer who grew wheat for his own consumption. The court concluded that his decision to grow — rather than purchase — wheat interfered with the government’s ability to regulate wheat prices.

The same logic should hold true for individuals able but unwilling to buy health insurance: Their absence has a significant impact on the market, especially because it is virtually inevitable that they will need health-care services at some point in their lives.

Tomorrow: If the court strikes down the mandate, must the rest of the law fall? Also, does the president’s health-care program unlawfully twist the arms of states to expand medical coverage to the poor?