The California Supreme Court is weighing a challenge to Prop. 8, the constitutional amendment banning gay marriage. If the Court upholds Prop. 8, one of the people you can blame is Attorney General Jerry Brown.

But wait a minute. Didn’t Brown make headlines recently by filing a brief in the Supreme Court arguing that Prop. 8 should be overturned? Yes. And that’s the problem.

In cases before the Supreme Court, it is the Attorney General’s job to defend California’s laws unless they are so plainly invalid that no plausible defense can be offered. However morally objectionable, Prop. 8 is not legally indefensible. Brown knows that. By switching sides in the Supreme Court, the Attorney General ripped up his job description– a political gambit that only makes it harder for the Court to overturn Prop. 8.

HOWEVER MORALLY OBJECTIONABLE, PROP. 8 IS NOT LEGALLY INDEFENSIBLE. BROWN KNOWS THAT.

The California Supreme Court is in a tough spot on Prop. 8. It went out on a limb a year ago to strike down a state statute forbidding same sex marriage, ruling that the law violated California’s constitution. Prop. 8, which voters enacted by a margin of 52 percent to 48 percent, responds directly to that controversial decision by amending the state constitution, thereby removing– or attempting to remove– the basis for the Court’s prior ruling.

The current Prop. 8 case poses a test of the Court’s legitimacy. Although its authority has never been greater, that authority derives from the public perception that the Court is above the political fray.

Should the Court strike down Prop. 8– overriding an electoral majority for the second time on the issue of same sex marriage– it must do so for reasons that are seen as legitimate and legally convincing, even though most Californians may disagree with the outcome. The Court must avoid the appearance that it is asserting a political preference disguised in legal principles. That is a tall order.

The last thing supporters of gay marriage need is a grandstanding attorney general who, by abandoning his assigned institutional role, provides an opening for Prop. 8 supporters to argue that the case has been transformed from a legal to a political contest in which victory goes to the most powerful interest groups.

Can the Court still overturn Prop. 8 in a way that will not compromise its legitimacy? I think so, although the best course at this stage may be a ruling grounded in the U.S. Constitution. Under a federal approach, Prop. 8’s problematic status as a state constitutional amendment loses relevance: vis a vis the U.S. Constitution’s equal protection guarantee, Prop. 8 is no different than any state statute or city ordinance.

Deciding the case on the basis of the federal constitution would legitimize the Court’s ruling– interpretation of federal law is part of its essential function in the original federal judicial scheme. Legitimacy also comes from the fact that the Court’s decision, if based on the U.S. Constitution, would not be the last word, but would be subject to review by the U.S. Supreme Court.

Of course, the availability of federal Supreme Court review is also the main disadvantage of this strategy. Still, the U.S. Supreme Court might decline to review the case, leaving in place a ruling blocking Prop. 8. Or it could review it and surprise everyone with a decision overturning Prop. 8. (Don’t underestimate the influence of the Supreme Court’s mostly liberal clerks on the gay marriage issue.) Let’s just hope Jerry Brown keeps his distance from any further judicial proceedings. With friends like Brown, Prop. 8’s opponents don’t need adversaries.

PETER SCHEER is executive director of the California First Amendment Coalition, a nonprofit advocacy group. www.cfac.org