For the first time, the U.S. Supreme Court has decided to tackle the quintessential civil rights issue of our era, and gay marriage advocates are downright giddy about it. Many appear to have convinced themselves that the justices — by a 5-4 vote, anyway — are gearing up for an historic '13 ruling that will sweep away all prohibitions in all 50 states, something akin to the historic '54 ruling against racial segregation.

But I wouldn't bet on that happening.

Yes, public opinion has shifted in recent years; thanks to the tolerant social attitudes of young people, a slim American majority now believes that marriage is a basic civil right open to all. But the high court generally prefers not to march too far ahead of public opinion. If history is any guide, the court next June will probably nudge the needle toward gay marriage - but in a split decision that stops far short of a happy ending.

Ever since the court agreed on Friday to wade into the issue - to determine whether the feds can rightfully deny federal benefits to gay couples who have been gay-married in states where the practice is legal; and even to determine whether gays have a basic U.S. constitutional right to marry - many progressive leaders have been suggesting that final victory is at hand. Elizabeth Birch, ex-director of the Human Rights Campaign, said yesterday, "We're living in an amazing era for gay people, and hopefully the Supreme Court will understand its moment in history." Chad Griffin, a key California activist, said that the court's willingness to weigh in "is nothing short of a milestone moment for equality."

Perhaps, but the court is generally slow on seeking out milestone moments. It took the justices until 1967 - after several decades of prodding from civil rights activists - to invalidate the 16 remaining state laws that barred interracial straight marriage. It took the justices until 2003 to invalidate the 13 remaining state laws that barred private gay sexual conduct - after having ruled during the '80s that those state sodomy laws were just fine.

But at least one gay rights victory next June is highly likely. The court seems poised to nix the noxious Defense of Marriage Act, the 1996 law, signed by President Clinton, that allows the federal government to discriminate against gay couples that have been legally wed under state law. Edie Windsor was married to Thea Spyer - their union was recognized by New York state - but after Spyer died in 2009, the widow was required to pay $363,053 in federal estate taxes. Windsor would've paid zip if she'd been wed to a male.

Windsor has already won her case against DOMA in federal appeals court. More importantly, Justice Anthony Kennedy, a Reagan appointee who frequently serves as the high court's swing vote, is a devotee of state's rights, and I can easily envision him teaming up with the four Democratic appointees to wave bye-bye to DOMA. That alone would be the biggest judicial win ever notched by gay rights supporters.

It's the other case that has the real milestone potential. I've written about it a few times, so I'll spare you the details again. California's voters barred gay marriage in a 2008 referendum, but the courts have since nixed the ballot result; basically, voter sentiment is trumped by the California constitution, which guarantees equal protection of the laws to all citizens. Anti-gay legal activists have appealed to the high court - and the lawyers for gay marriage are fine with that, because they're prepared to argue next March that the U.S. document, in its language mandating equal protection of the laws, gives all citizens "a constitutional right to marry."

That quote is from key player Theodore Olsen, the conservative legal scholar and ex-Bush solicitor general. He'll ask the justices to issue a milestone ruling that will sweep away the prohibitions in 30 states that bar gay marriage. This is the judicial payday that gay marriage advocates are pining for. They also seem to believe that the justices are poised to think big in order to stay hip with public opinion; in the words of Evan Wolfson, director of Freedom to Marry, "The key is to create a climate of encouragement for the court...by winning even more hearts and minds. We want the justices to see that (a) ruling for the right side of history will not only stand the test of time, but be true to where the American people are."

But the court may not want to be so sweeping, because plenty of "the American people" live in red states with prohibitive state laws, and a milestone ruling would be tantamount to an earthquake. It seems far more likely that the court will find a way to narrow this case - decreeing, for instance, that the California constitution does indeed trump the California ballot measure, but insisting that the decision applies only to California - rather than go big with a ruling that nationalizes gay marriage. Although it seems inevitable that such a day will come. Just not yet, if the court's past behavior is any indication.

But here's some perspective. Back in the mid-1960s, U.S. immigration authorities rejected a Canadian, Clive Michael Boutilier, who had applied for citizenship. Boutilier, who had been living in New York, was summarily deported back to Canada. The reason: He was gay. And under the provisions of a 1952 federal immigration law, gays were inherently defined as "persons afflicted with psychopathic personality." Boutilier, still intent on becoming an American, took his case all the way to the Supreme Court. He lost. The justices explained that his case hinged on "a purely legal question." He was gay, the law defined him as "psychopathic" - and that was good enough for the justices.

So whatever happens next June, think how far we've already come.

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Is there a "second-term curse," and how can Obama avoid it? My Sunday newspaper column.

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I'm interviewing Bob Woodward tonight, at a public event at the Philadelphia Free Library. If you have a good question for him, email it to me. Use the address at the top of this page.

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