A healthy 20-something might ask: Can the government really order me to spend more than $5,500 a year to buy comprehensive health insurance just because I live in the United States, even though the most I might need or want is catastrophic coverage costing less than $800?

Can they really force me to pay a big penalty "tax" if I won’t buy government-approved insurance? And can they use my money to subsidize people who are twice my age, and obese or sick, even if they have more money than I do?

The answers are yes, yes, and that’s the point! according to most of the experts who have weighed in on whether the Supreme Court would uphold a mandate for individuals to buy comprehensive health insurance unless they’re already covered by employer-based plans. They cite the justices’ very broad reading since the New Deal of Congress’s powers to regulate interstate commerce and to tax and spend.

But a hardy band of libertarian and conservative lawyers and scholars has been attacking the mandate’s constitutionality with gusto, as have congressional Republicans led by Sen. Orrin Hatch, R-Utah. He made a forceful case in leading off a December 9 Heritage Foundation panel.

Never in the history of the nation, these experts stress, has Congress adopted a law requiring people to buy a product or service simply because they exist and live in the United States. Nor has the Supreme Court held that Congress can penalize inactivity in the name of regulating interstate commerce.

Such a law would offend the libertarian streak in many Americans. It seems a stretch to see it as within Congress’s power to "regulate commerce … among the several states."

Although the Constitution also grants Congress power "to make all laws which shall be necessary and proper" to regulate interstate commerce, and some precedents provide indirect support for a health insurance mandate, no precedent squarely supports it.

Indeed, the Congressional Research Service, in a July 24 analysis of whether a mandate would be constitutional, cautioned that it is a "challenging" and "novel" question whether Congress may use the commerce clause "to require an individual to purchase a good or service."

So, why are most experts (and this columnist) so sure that the justices would uphold the mandate? And why do even Washington lawyers David Rivkin and Lee Casey — the most prolific and among the most cogent critics of the mandate’s constitutionality — stop short of predicting that the Court would strike it down?

The answers shed light on how far constitutional jurisprudence has devolved — inevitably, in my view — into a mix of policy preferences, political prudence, and legal precedents that long ago departed from the actual words and original meaning of the Constitution.

Consider the views of Georgetown law professor Randy Barnett, a leading libertarian, who spoke at the Heritage panel. He came close to predicting that the justices would strike down "a program that may well be very unpopular politically by the time a challenge reaches the Court," as he put it the same day in The Volokh Conspiracy, a leading legal blog.

In an earlier exchange on the same blog, a colleague’s suggestion that the justices would not pick a massive fight with a still-popular president backed by a large congressional majority provoked this biting retort from Barnett.

"So what ‘constitutionality’ really comes down to is whether five justices have the, er, nerve to strike down a popular act of popular Congress…. But how about a not-so-popular act of a not-so-popular Congress by the time the case reaches the high court? What if the Repubs take back the Congress by then? … And if, to assess its constitutionality, we have to calibrate the popularity of the law and/or lawmaker … does this tell us anything about the constitutional law game in our fair Republic?"

That’s too cynical a spin, for my taste, to put on the justices’ undoubted sensitivity to public opinion and political considerations.

It’s true that the proposed mandate, like much else that Congress has done since the New Deal, would extend federal powers far beyond anything envisioned by the Framers. The commerce clause, in particular, was not intended to allow Congress to regulate activities that were neither interstate nor commercial.

But the need to govern an ever-more-interconnected nation, in which once-local activities such as health care have become critical components of the national economy, has spawned a long line of precedents expanding the commerce power, especially since the justices began upholding New Deal programs in 1937.

On this and other issues, the Court must often choose whether to be faithful to a literal interpretation of the Constitution’s original meaning or to the hundreds of precedents that — sometimes for good policy reasons, sometimes not — have stretched or departed from it.

The strongest precedents for supporters of the mandate are Gonzales v. Raich, a 6-3 decision in 2005 that Congress can make it a federal crime for Californians to grow marijuana at home for their own personal, medical use — even though this is legal as a matter of California law — and Wickard v. Filburn, a 1942 ruling that Congress could limit the amount of wheat that farmers grew to feed their own livestock.

Homegrown-and-consumed marijuana and wheat don’t sound much like interstate commerce. But Raich held that it is "necessary and proper" for Congress to regulate local behavior when doing so is "an essential part of a larger regulation of economic activity, in which the regulatory scheme could be undercut unless the intrastate activity were regulated."

Inactivity — failing to buy health insurance — is even further removed from interstate commerce than the medical marijuana in Raich. But uninsured people affect commerce, and in many cases impose costs on society, when they seek free care from emergency rooms or spread communicable diseases. The proposed mandate can also be defended as "necessary and proper" to finance the subsidies that are the core of the Democratic health reform effort, as Eugene Volokh, a law professor at UCLA, stressed at the Heritage panel.

(Yale Law School’s Jack Balkin and others assert that Congress’s authority to tax and spend for the general welfare is even more sweeping than the commerce power, and more than sufficient to support a penalty tax for failing to buy health insurance. But this argument has gotten less attention than the commerce clause, on which the Senate has based the mandate itself.)

Rivkin and Casey counter that the justices have never held congressional impositions on people who engage in no relevant activity at all to be necessary and proper to regulation of interstate commerce. Such a holding would cross an important line, and would make America a little bit less free, they say quite persuasively. The same logic could support a mandate to buy health club memberships, cars (to stimulate the economy), or anything else, they add.

The most critical point, Rivkin and Casey say, is that the commerce power must be limited by some coherent principle, under the logic of the 5-4 decisions in 1995 and 2000 striking down a federal law against possessing a gun near a school and a provision of the Violence Against Women Act. And no such principle could survive a decision upholding a health insurance mandate.

I don’t see a clear winner in these dueling arguments about commerce-clause precedent. I also think that it would be far better for Congress simply to increase taxes to finance the subsidies, which — while politically toxic — would raise no constitutional problem.

But I am nonetheless fairly confident that the justices would, and should, defer to the political branches here. The alternative would be to strike down the president’s signature initiative — something that no Court has done in more than 70 years, for good reason.

But what if, as Barnett predicts, most voters end up with buyer’s remorse? What if more and more of them come to see the legislation as — in the words of Washington Post columnist Robert Samuelson — an "economic assault on the young by the old," whose established entitlement programs "darken the young’s already poor economic prospects?" What if a backlash fuels a Republican sweep of the 2010 elections? And what if — politics aside — the whole business comes to look like a mess that can be salvaged only by a Supreme Court decision clearing the decks for Congress to rethink health care reform from the ground up?

Such are the dreams of those who imagine the justices striking down the proposed health care mandate. I can only hope that they don’t all come true. There are worse things than making America a little bit less free by way of deeply flawed health care legislation. And one of them would be an all-out war between the Supreme Court and the elected branches.