Actually, according to the Associated Press, Judge Joseph Tauro based his ruling on two separate but linked cases on the Tenth and Fourteenth Amendments.  In overturning the Defense of Marriage Act (DOMA), Tauro ruled that the law interfered with state rights — a specific 10th Amendment issue — to define marriage:

A U.S. judge in Boston has ruled that a federal gay marriage ban is unconstitutional because it interferes with the right of a state to define marriage.

U.S. District Judge Joseph Tauro on Thursday ruled in favor of gay couples’ rights in two separate challenges to the 1996 Defense of Marriage Act, known as DOMA.

The state had argued the law denied benefits such as Medicaid to gay married couples in Massachusetts, where same-sex unions have been legal since 2004.

Tauro agreed, and said the act forces Massachusetts to discriminate against its own citizens.

“The federal government, by enacting and enforcing DOMA, plainly encroaches upon the firmly entrenched province of the state, and in doing so, offends the Tenth Amendment. For that reason, the statute is invalid,” Tauro wrote in a ruling in a lawsuit filed by Attorney General Martha Coakley.

The ruling in the paired case rests on the equal-protection clause.  Tauro apparently wants to cover all his bases for the inevitable appeal.

The 10th Amendment application seems a little odd to me, especially in the case of Medicaid coverage.  That program uses federal funds in part to cover medical bills.  The federal government would therefore seem to have jurisdiction on how its own funds get spent, although the state should have the same latitude with its own funds.  After all, DOMA doesn’t tell states that it can’t recognize same-sex marriages, but just exempts marriage recognition from the full faith and credit clause of the Constitution so that other states don’t have to follow suit.  It also retains federal jurisdiction on marriage definition for the purpose of spending federal money on partner benefits, which also has nothing to do with the 10th Amendment.

If the Supreme Court endorses this stand, though, it sets up an interesting question for conservatives who express support for better enforcement of the 10th Amendment.  Can they get behind this interpretation?  And will this sudden interest in applying the 10th Amendment by the judiciary start spreading to other issues, especially in rethinking a century’s worth of decisions on the commerce clause?