Once upon a time, the health-care debate was political. It was messy and mean and occasionally absurd, but it made progress, and the result was an imperfect but historic law justifiably seen as President Barack Obama’s legacy.

Today, the legal debate over health care reaches the Supreme Court. But not only is the president’s legacy on trial, so is the court’s. Almost as important as what the court says -- and we think the case against the law is pretty thin -- will be how it says it: The purpose of the judicial branch is to provide guidance and clarity to the other two branches. A 5-to-4 decision will not do that.

The main issue before the court, and the topic of two of the unprecedented six hours of oral argument the court will hear over the next three days, is the individual mandate. This is the requirement that Americans either have health insurance or pay a penalty. Challengers to the law, which include 26 states and the small-business lobby, argue that Congress lacks the authority to impose such a mandate. It is, as the aggrieved states say 19 times in their brief, unprecedented.

Not Unprecedented

Unprecedented is a good word to throw around if you want to get people’s attention, but it does not a legal argument make. Let’s take a step back: One of the main purposes of the (uppercase) Law is to explain how almost every case is, for lack of a better term, precedented. Every Supreme Court opinion for at least the last 200 years has been an exercise in comparative analysis, explaining how the controversy it is now considering is a lot like one it has seen before. Of course, every case presents new issues -- if it didn’t, the court wouldn’t agree to hear it -- but they are never as novel or (that word again) unprecedented as the litigants would have the court believe.

But enough about the Law; what about this law? How does the individual mandate compare to previous exercises of congressional power?

The power is derived from the Commerce Clause of the Constitution, which gives Congress the authority to “regulate commerce … among the several states.” The question is what qualifies as interstate commerce. For most of the second half of the 20th century, the answer has been clear -- and forgive us if we lapse into legal jargon: pretty much anything.

Is a farmer growing wheat for his own consumption engaging in interstate commerce? Yes. A small restaurant in Alabama refusing to seat blacks? Yes. A sick Californian growing her own medicinal marijuana, as allowed by state law? Yes. And so on.

Fine, say opponents of the law. We can’t help it if the court has interpreted the Commerce Clause too broadly. They point to a few more recent cases, involving laws about guns near schools and violence against women, in which the court has said Congress overstepped its bounds. They also argue, not without reason, that Congress’s power under the Commerce Clause cannot be completely without limits.

Here is where they invoke broccoli. If Congress can force me to buy health insurance, they ask -- and this analogy has actually appeared in a federal judicial opinion -- can’t it also force me to eat broccoli?

In a word (or two): Yes and no. Congress could certainly pass a law requiring the consumption of broccoli. And the courts would certainly invalidate it. Without disputing the point that the Constitution grants the U.S. government only limited powers, it is easy to conclude that Congress is well within its authority to enact the individual mandate.

Not Broccoli

Every American, by virtue of his or her existence, is a consumer of health care. The same, sadly, cannot be said for broccoli. Health care accounts for about 18 percent of the economy, and an even larger share of the federal budget. Hospitals and insurance companies and their affiliates operate and treat patients on a national scale and across state lines. Precise figures are unavailable, but broccoli’s role in the U.S. economy is considerably smaller.

The power to regulate interstate commerce also comes with the power (or so the Supreme Court said 188 years ago) to choose “the rule by which commerce is to be governed.” The individual mandate is that rule.

This is not, when all is said and done, a hard case. Which raises two final questions: How did it ever reach the Supreme Court? And why is the court spending so much time on it?

To a certain extent the lower courts, with their divergent opinions on the case, forced the Supreme Court’s hand. As for why the court has decided to spend three days on the case, no one can say for sure. One charitable answer is that the court realizes this issue is divisive and wants to ensure that all sides feel they have been heard. (Arguing in favor of this theory is the fact that the court has asked for briefs on questions neither the administration nor the states dispute.)

The less charitable answer brings us back to where we started. The politics of health care date back not just three years or two decades but at least a century, and the politics of the courts are increasingly stark. Politics certainly played a role in opposition to the law, which passed along purely partisan lines. And the ink was barely dry on the president’s signature when the first suit against the law was filed on March 23, 2010.

The law is supposed to be above politics, of course -- though we could as easily say that our political debates needn’t be confined by the law. In the debate over health care in the U.S. -- how to extend it to more people, how best to pay for it, how to make it more fair and efficient -- most of the points of contention are not constitutional.

That larger discussion will outlast the one the Supreme Court is holding this week. The best contribution the court can make is a strong, lucid decision in favor of the law. Then the health-care debate can become, once again, political.

Read more opinion online from Bloomberg View.

To contact the Bloomberg View editorial board: view@bloomberg.net.