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."