Would a law requiring Americans to eat their fruits and vegetables be constitutional? During Elena Kagan's Supreme Court confirmation hearings in June 2010, Sen. Tom Coburn asked her whether such a law could be justified as an exercise of the federal government's authority to "regulate commerce…among the several states." When Kagan dodged the question, the Oklahoma Republican pressed her: "What if I said that eating three fruits and three vegetables [a day] would cut health care costs 20 percent? Now we're into commerce. And since the government pays 65 percent of all the health care costs, why isn't that constitutional?"

Kagan never did answer the question, except to say that a "dumb law" is not necessarily unconstitutional, which was hardly reassuring to those who suspected that her reading of the Commerce Clause was broad enough to accommodate the Patient Protection and Affordable Care Act's requirement that Americans buy government-approved health insurance. But Coburn, despite his professed concern about Kagan's "expansive view of the Commerce Clause," has been known to read that authority rather broadly himself. The senator brags about writing the Partial-Birth Abortion Ban Act, a 2003 law that makes it a federal crime to use a specified abortion technique "in or affecting interstate or foreign commerce."

As Independence Institute Research Director David Kopel and University of Tennessee law professor Glenn Reynolds noted in a 1997 Connecticut Law Review article, "it is not really possible to perform an abortion 'in or affecting interstate or foreign commerce'…unless a physician is operating a mobile abortion clinic on the Metroliner." Such language, they argued, can only baffle "any person not familiar with the Commerce Clause sophistries of twentieth century jurisprudence." Those are precisely the sophistries that Coburn claims to resist but is happy to deploy when they advance his purposes.

It has always been thus, University of Oregon law professor Garrett Epps argued in a 2001 New York Times op-ed piece. "When it comes to states' rights," Epps wrote, "we are all hypocrites." His immediate example was John Ashcroft, who as governor of Missouri had decried congressional dreams of "an all-powerful centralized bureaucracy" but as George W. Bush's attorney general tried to stop Oregon from implementing its Death With Dignity Act. Epps added that Thomas Jefferson, James Madison, Stephen A. Douglas, and Abraham Lincoln all displayed similar inconsistencies. "One scans American history in vain to find a major figure whose position on states' rights was not directly connected to his or her position on the underlying political question," Epps wrote. "When it suits our leaders, they are in favor of broad federal power; when it does not, they claim 'states' rights.'"

A Grand Bargain

In principle, at least, it does not have to be this way. Precisely because federalism is useful to people with different political agendas, it should be possible to strike a deal: I will not use the federal government to interfere with your local policy choices if you grant me the same leeway. That's assuming states do not try to exercise powers prohibited by the Constitution (and there is room for disagreement about precisely what those are). But broadly speaking, Americans have an interest, regardless of their political affiliations, in making sure that Congress exercises only those powers "delegated to the United States by the Constitution," as the 10th Amendment puts it, with the rest "reserved to the States respectively, or to the people."

Unfortunately, for most progressives federalism is linked not just to conservatism but to reactionary racism. "In the Tea Party's talk of states' rights," New York Times reporter Kate Zernike wrote in 2010, "critics say they hear an echo of slavery, Jim Crow and George Wallace." Last year in Mother Jones, Josh Harkinson listed Ron Paul's devotion to federalism as one of his "15 most extreme positions." He did not mean that as a compliment. Harkinson said the Texas congressman, if elected president, would "enable state extremism" by letting states "set their own policies on abortion, gay marriage, prayer in school, and most other issues."

Despite the left's general hostility to federalism, there was a flurry of commentary about the possibility of using the principle for progressive ends right after the 2004 elections, when George W. Bush won a second term and the Republicans bolstered their majorities in the House and Senate. "Having lost any say in how the nation is run, liberals may be about to discover states' rights," Michelle Goldberg wrote in Salon two weeks after Election Day. Goldberg cited gay marriage, health coverage, gun control, and abortion as areas where progressives could advance their agenda at the state level. "A push to make abortion illegal nationwide," she said, "would leave pro-choice states relying on the doctrine of federalism, or states' rights, to defend themselves."

That scenario never transpired, but it illustrates the risks of nationalizing abortion policy via Roe v. Wade, the 1973 decision in which the Supreme Court discovered a constitutional right to abortion. After arguing for years that regulation of the procedure should be left to the states, conservatives began using the federal government to impose restrictions, such as Coburn's ban on "partial birth abortion." Making abortion a national issue raised the stakes and the level of rancor, so much so that abortion rights supporters might have been better off pushing their preferred policies state by state.

Two Supreme Court cases decided during George W. Bush's second term further illustrate how federal involvement can jeopardize progressive causes. In Gonzales v. Raich (2005), the Court ruled that the Commerce Clause authorizes the federal government to enforce its absolute ban on marijuana even in states that allow medical use of the plant and even against patients who grow their own. The decision arguably went even further than Wickard v. Filburn, the 1942 ruling that said Congress has the authority to stop a farmer from growing wheat for his own use because such self-reliance reduces aggregate demand, thereby exerting "a substantial economic effect on interstate commerce."

Justice John Paul Stevens, writing for the majority in Raich, said the Commerce Clause applies even to homegrown marijuana because of "the likelihood that the high demand in the interstate market will draw such marijuana into that market." Making exceptions for state-authorized medical use of marijuana, Stevens wrote, would undermine the "comprehensive regulatory regime" established by the Controlled Substances Act. In a concurring opinion, Justice Antonin Scalia said Congress "may regulate even noneconomic local activity if that regulation is a necessary part of a more general regulation of interstate commerce." Not surprisingly, the Obama administration embraced Scalia's argument, saying the individual insurance mandate is an essential component of its health care regulations.

The other Supreme Court decision suggesting that progressives should reconsider their resistance to federalism also dealt with the Controlled Substances Act. In the 2006 case Gonzales v. Oregon, the Court ruled that prescribing barbiturates to terminally ill patients who want to kill themselves, as Oregon allows, is a permissible use of controlled substances under federal law. It thereby overruled Attorney General Ashcroft, who had threatened to prosecute Oregon doctors who assisted suicides. Although the decision hinged on statutory rather than constitutional interpretation, Justice Anthony Kennedy noted in his majority opinion that "the statute manifests no intent to regulate the practice of medicine generally," adding that "the silence is understandable given the structure and limitations of federalism."

Citing the Bush administration's attempts to override state policies regarding medical marijuana and assisted suicide, Michelle Goldberg noted that "many conservatives stop advocating for states' rights as soon as they get their hands on the levers of federal power." Not just conservatives. After the Democrats took control of Congress in 2006 and Barack Obama won the White House in 2008, the left's interest in federalism seemed to wane, with a few notable exceptions.

One is the Constitutional Accountability Center, a Washington, D.C., think tank "dedicated to fulfilling the progressive promise of our Constitution's text and history." The center is especially interested in Supreme Court decisions that give states freedom to set their own regulatory policies and apply their own tort law. In the 2009 case Wyeth v. Levine, for example, the Court ruled that a Vermont woman who lost an arm as a result of a botched injection could recover damages from the drug's manufacturer in state court based on inadequate labeling, even though the company's instructions passed muster with the U.S. Food and Drug Administration. Last year in Williamson v. Mazda Motor, the Court likewise said the family of a woman who died in a traffic accident could sue the car's manufacturer in California court for failing to install shoulder belts that were encouraged but not required by the federal government.

Noting that Justice Clarence Thomas had written concurring opinions in both cases, Elizabeth B. Wydra, general counsel of the Constitutional Accountability Center, welcomed his "emergence as a surprising ally for progressives." In Wyeth, Thomas criticized the "implied pre-emption" doctrine, under which "the Court routinely invalidates state laws based on perceived conflicts with broad federal policy objectives, legislative history, or generalized notions of congressional purposes that are not embodied within the text of federal law." Arguing against federal pre-emption in Williamson, he said "the Supremacy Clause commands that the '[l]aws of the United States,' not the unenacted hopes and dreams of the Department of Transportation, 'shall be the supreme Law of the Land.'"

One-Way Federalism

Not coincidentally, Thomas, who has repeatedly criticized the "substantial effects" doctrine, was considered the surest vote against the health insurance mandate. (Despite Wydra's gratitude for Thomas' consistent federalism, she did not return the favor in the ObamaCare case, arguing that the Court should uphold the mandate.) Thomas also was one of three dissenters in Raich, the medical marijuana decision. "If Congress can regulate this under the Commerce Clause," he declared, "then it can regulate virtually anything—and the Federal Government is no longer one of limited and enumerated powers."

Despite that setback, left-leaning drug policy reformers continue to use federalist arguments, demanding that Washington let states experiment with more tolerant approaches. Last year Americans for Safe Access filed a federal lawsuit arguing that the Obama administration's crackdown on medical marijuana in California violates the 10th Amendment. The group says the Justice Department is not simply enforcing the federal ban on marijuana (as permitted by Raich) but doing so in a way intended to undermine state and local policy choices. "While the government is entitled to enforce its criminal laws against marijuana…in an even-handed manner," the complaint says, "the Tenth Amendment forbids it from selectively employing such coercive tactics to commandeer the law-making functions of the State."

Federalism also can serve the cause of gay rights. In 2010 U.S. District Judge Joseph Tauro ruled that the Defense of Marriage Act (DOMA), which prohibits the federal government from recognizing state-approved gay marriages, violates the 10th Amendment. When it comes to joint federal-state projects such as Medicaid and military cemeteries, Tauro said, DOMA forces states like Massachusetts to pretend that marriages they recognize do not exist, a requirement that impermissibly intrudes on family law, "a quintessential area of state concern."

Bob Barr, who came to regret his role in writing DOMA as a Republican congressman from Georgia, originally viewed the 1996 law as a way to prevent one state from imposing its marriage policies on others. But as he explained in a 2009 Los Angeles Times op-ed piece, "DOMA's language reflects one-way federalism: It protects only those states that don't want to accept a same-sex marriage granted by another state. Moreover, the heterosexual definition of marriage for purposes of federal laws—including immigration, Social Security survivor rights and veteran's benefits—has become a de facto club used to limit, if not thwart, the ability of a state to choose to recognize same-sex unions."

In this context, Josh Harkinson's concern that Ron Paul's federalism would "enable extremism" by letting states set their own marriage policies is puzzling. As Yale law professor Heather K. Gerken observes in the Spring 2012 issue of the journal Democracy, local and state decisions to recognize gay marriage have helped shape public opinion by disproving social conservatives' predictions of disaster and showing the nation "happy families that looked utterly conventional save for the presence of two tuxedos or two wedding dresses." Gerken advocates "a new progressive federalism," arguing that "state and local governments have become sites of empowerment for racial minorities and dissenters" in areas such as marriage, education, affirmative action, immigration, and environmental regulation. "Decentralization will produce policies that progressives adore," Gerken writes, "and it will produce policies that they loathe." But she argues that the risks for progressives are outweighed by the benefits, including not only instructive policy experiments but a stronger democracy that integrates outsiders into the political system.

Just as federalism will not always produce policies that progressives like, it will not always produce policies that libertarians like. Federalism, like representative democracy or the federal government's "checks and balances," is a means to an end: By keeping most political decisions at the state and local levels, it promotes responsiveness, diversity, innovation, and competition among jurisdictions. "When political power is decentralized," notes George Mason law professor Ilya Somin in a 2011 Library of Law and Liberty essay, "individuals can 'vote with their feet' against jurisdictions whose policies are oppressive or heavy-handed." People who are unhappy with the policies of one city or state can move to another, much more readily than they can immigrate to a different country.

Federalism therefore is no mere formality, and it is not simply about "states' rights." As Justice Kennedy observed in Bond v. U.S., a unanimous 2011 decision allowing criminal defendants to challenge federal charges on 10th Amendment grounds, "The federal system rests on what might at first seem a counterintuitive insight, that 'freedom is enhanced by the creation of two governments, not one.'…The Framers concluded that allocation of powers between the National Government and the States enhances freedom, first by protecting the integrity of the governments themselves, and second by protecting the people, from whom all governmental powers are derived."

For this system to work, however, the distinction between state and federal powers must be determined by the Constitution, not by personal policy preferences. A consistent application of federalism is not just a matter of principle or intellectual honesty. It is a matter of pragmatism, since the same understanding of congressional power that allows the federal government to pursue universal health care also allows it to yank medical marijuana from the hands of cancer patients.

Senior Editor Jacob Sullum is a nationally syndicated columnist.