When President Obama endorsed gay marriage last year, he said the issue should be left to the states. Last week he said it shouldn't.

To be more precise, a Supreme Court brief filed by the Obama administration last Thursday argues that California's ban on gay marriage denies homosexuals the "equal protection of the laws" guaranteed by the 14th Amendment. Although the brief focuses on Proposition 8, the 2008 ballot initiative that overturned a California Supreme Court decision requiring the state to recognize gay marriages, its logic suggests that a policy Obama himself rejected less than a year ago is constitutionally mandatory.

For many years Obama said he supported equal rights for gay couples, except for the right to call their relationship a "marriage." That is exactly the policy he now says is unconstitutional.

Proposition 8 amended California's constitution to declare that "only marriage between a man and a woman is valid or recognized." But the initiative's backers assured voters that "Proposition 8 doesn't take away any rights or benefits of gay or lesbian domestic partnerships."  And under California law, as the Obama administration's brief notes, "domestic partnerships carry all the substantive rights and obligations of marriage."

The administration argues, rather counterintuitively, that California's decision to treat gay and straight couples the same but for the word marriage makes its policy more vulnerable to constitutional challenge than a policy that does not recognize same-sex unions at all. Why? Because the only point of withholding the label is to mark gay marriages as inferior, a goal motivated by "impermissible prejudice," which is not a constitutionally valid reason for treating people differently under the law.

The administration could have argued, as the trial judge and the U.S. Court of Appeals for the 9th Circuit did, that the justification for Proposition 8 is so slight that it fails even the highly deferential "rational basis" test that is used in most equal protection cases. But the Justice Department had already taken the position, in a separate case involving the federal Defense of Marriage Act (DOMA), that discrimination based on sexual orientation should receive the same sort of "heightened scrutiny" that the Supreme Court has said is appropriate for discrimination based on sex or "illegitimacy" (i.e., birth outside of marriage).

To withstand heightened scrutiny, a legal distinction must substantially further an important government interest. The Justice Department, which announced two years ago that it would no longer defend DOMA and is now actively opposing it before the Supreme Court, concluded that the statute's ban on federal recognition of state-approved marriages between people of the same sex fails that test, since it affects marriage policy only "at the margin."

Likewise, says Solicitor General Donald Verrilli, Proposition 8 does not substantially advance any legitimate interest, since its impact is almost entirely symbolic. His brief strongly implies that the same argument invalidates the laws of seven other states (Delaware, Hawaii, Illinois, Nevada, New Jersey, Oregon, and Rhode Island) that give same-sex couples "rights substantially similar to those available to married couples, yet still restrict marriage to opposite-sex couples."

Nine states and the District of Columbia recognize gay marriage. So what about the remaining 33? It is pretty clear from Verrilli's discussion of the arguments for banning gay marriage that the administration does not think those states' laws could survive heightened scrutiny either.

But if the Supreme Court, which is scheduled to hear this case on March 26, adopts the administration's reasoning, the decision could discourage states from moving toward recognition of gay marriage, because doing so would make their laws less substantive and therefore less likely to be upheld. That would be a bizarre result, since it is hard to understand how giving gay couples none of the rights and privileges associated with marriage is less offensive to the principle of equal treatment under the law than giving them all those rights and privileges while calling their relationship something else.