If the Supreme Court strikes down some or all of the Affordable Care Act (ACA) this month, the negative consequences will be several and serious.

The ACA guarantees access to affordable health insurance to roughly 40 million Americans who otherwise could not afford it — people who do not obtain insurance through their employer, Medicare, or Medicaid. If the court invalidates the ACA, someone who loses a job (and thus her insurance) and becomes ill is in financial peril. She likely will be denied new coverage based on her preexisting condition.

It would be one thing if our Constitution prohibited Congress from addressing this awful situation with the ACA. But this is exactly the kind of problem that our Constitution empowers Congress to solve.

Since the 1930s, the Supreme Court has recognized that the Constitution gives Congress robust authority to address interstate economic problems. Because our country has open interstate borders and mobile participants in health care and insurance markets, states acting alone are not well situated to require that insurers cover people with preexisting conditions. Insurers can move to states that don't impose such a requirement. Moreover, individuals may have to turn down better jobs in other states because they can't afford to lose their health insurance.

The ACA's opponents nonetheless seize on the law's minimum-coverage provision, which requires most Americans to either obtain a minimum level of health insurance or pay roughly $700 each year. Opponents insist that Congress' powers are limitless if the court upholds it.

They are wrong. By solving interstate economic problems, the minimum coverage provision respects crucial limits on Congress. And the provision violates no constitutional rights, as would forcing people to eat broccoli.

In fact, defensible limits are lacking not in the federal government's arguments, but in those of the ACA's opponents.

For example, opponents assert that Congress' commerce power does not support the minimum-coverage provision because Congress may not use this authority to regulate inactivity (not buying insurance). Take this novel suggestion seriously and Congress may not mandate vaccination to combat a deadly flu pandemic spreading around the country. Given the mobility of Americans, we would all be at the mercy of a state that refused to mandate vaccination on libertarian grounds.

Our Constitution is not a suicide pact.

Or consider another justification for the minimum-coverage provision, the "necessary and proper clause." Opponents agree that Congress may use the commerce clause to require that insurers cover people with preexisting conditions. Under long-standing doctrine, the minimum-coverage provision is necessary and proper to carrying into execution this concededly constitutional regulation of insurers. The provision combats the perverse incentive people would otherwise have to wait until they are sick to obtain insurance, thereby unraveling insurance markets.

Opponents argue that this is inappropriate because it would give the federal government too much power. But it has long been settled that Congress must be able to make its regulations of interstate commerce effective. Congress' plan for prohibiting insurers from denying coverage because of preexisting conditions won't work without the minimum coverage provision. Our Constitution gives Congress the authority it needs to protect its citizens.

If the court disagrees and imposes the above limits on Congress to invalidate the minimum-coverage provision, it will severely compromise congressional power to regulate many problems.

If the court instead invalidates the minimum-coverage provision but doesn't impose these limits on other federal laws, the decision will be unprincipled.

And that suggests another distressing consequence of invalidating the ACA — one for the court itself. The court may be hard-pressed to dispel the perception that it is partisan.

The Republican Party made obstruction of the health-care bill a central political strategy during the first two years of Barack Obama's presidency. After the bill passed, Republicans then objected that the minimum-coverage provision — originally promoted by Republican politicians and conservative think tanks — was unconstitutional, in an effort to secure the victory in court they couldn't obtain in politics. This strategy flouts traditional conservative arguments for judicial restraint, which helps explain why the objection has been rejected by some of our nation's most respected legal conservatives, including Charles Fried, Henry Monaghan, Laurence Silberman, Jeffrey Sutton, and J. Harvie Wilkinson III.

Striking down the ACA would amount to the most consequential — and least justified — invalidation of a federal law on federalism grounds since the Great Depression.

E-mail Neil S. Siegel at siegel@law.duke.edu.