In 1995, the U.S. Supreme Court overturned a federal law that banned gun possession near schools. For the first time since the New Deal, the court ruled that Congress had exceeded its constitutional authority to regulate interstate commerce, its usual excuse for meddling in state and local matters.

A year later, Congress passed the same law again, this time specifying that a defendant can be found guilty of carrying a gun in a school zone only if the weapon "has moved in" or "otherwise affects" interstate or foreign commerce. While 72 of his fellow senators pretended to believe this easily satisfied requirement rendered the law constitutional, Fred Thompson voted against the transparent ruse.

It was not the only time Thompson, now a candidate for the Republican presidential nomination, found himself on the losing end of a lopsided vote to assert authority Congress does not have. With some notable exceptions, the Tennessee Republican's Senate record suggests he may be that Washington rarity: a politician who means what he says, at least when it comes to the division of powers between the federal government and the states.

As Thompson emphasizes, this division is crucial to our system of government: By avoiding an all-powerful central authority, it protects liberty, promotes accountability and fosters competition that leads to innovation and diversity in public policy. Recognizing that the federal government has only those powers specifically enumerated in the Constitution, Thompson says, "Folks in Washington ought to be asking first and foremost, 'Should government be doing this? And if so, at what level of government?'"

Although he does not always come up with the right answers, Thompson does seem to ask himself these questions. He brags about casting the lone Senate vote against popular measures that impinged on state or local prerogatives, including bills shielding teachers and Good Samaritans from civil liability and an amendment urging schools to adopt "zero tolerance" policies against violence and drug use.

Thompson also broke ranks with fellow Republicans by opposing federal attempts to limit medical malpractice awards, cap punitive damages in product liability cases, and restrict lawyers' fees, arguing that such issues should be left to the states. In 1998, he was one of 32 senators who voted against an amendment aimed at establishing a national legal standard for drunk driving.

Thompson is no Ron Paul, and he has deviated from his avowed principles on more than a few occasions, including his support for President Bush's expansion of federal involvement in education. The biggest challenge to Thompson's federalism probably has been the temptation to support socially conservative measures that exceed congressional authority.

Thompson backed a federal ban on human cloning and voted repeatedly to prohibit "partial birth" abortion, under the same absurd pretext he rejected in the context of the Gun-Free School Zones Act: The abortion law, which was enacted in 2003, applies to abortions "in or affecting" interstate commerce.

Regarding gay marriage, Thompson has stuck closer to his constitutional principles, opposing a federal ban and saying the matter should be decided by state legislatures. But he recently muddied the waters by advocating a constitutional amendment that would bar state judges from requiring state legislatures to permit same-sex unions.

A more hopeful sign is the Thompson campaign's unequivocal statement that Congress overstepped its bounds in 2005 by intervening in the case of Terri Schiavo, the Florida woman in a persistent vegetative state whose feeding tube her husband wanted to disconnect, contrary to her parents' wishes. A decent respect for federalism also requires that Thompson oppose the Bush administration's efforts to override state policies regarding the medical use of marijuana, an issue on which he has not taken a stand.

As he chooses between the demands of the Constitution and the demands of social conservatives, Thompson should keep in mind his own warning. Federalism, he wrote last spring, "is something we all give lip service to and then proceed to ignore when it serves our purposes."