Gay Rights May Get Its Brown v. Board of Education OF the many momentous issues the Supreme Court will take up this term, few are as volatile as same-sex marriage. In the term that began last week, the justices are expected to consider the constitutionality of the federal Defense of Marriage Act, and possibly state bans on gay marriage as well. The question looms: how might they rule? The Defense of Marriage Act, or DOMA, the 1996 law denying federal recognition to same-sex marriages lawfully performed by states, is probably the easier case for the justices. Historically, Congress has deferred to state definitions of marriage, and several federal courts have invalidated the law on these grounds of federalism. DOMA is also vulnerable because some of its legislative sponsors defended it in blatantly homophobic terms. Polls show that a substantial majority of Americans are opposed to it. Conservative justices who value federalism and liberal justices who sympathize with gay marriage will probably combine to invalidate the act. Predicting how the justices might rule on state bans on same-sex marriage is harder, as the relevant legal doctrine - which flows from judicial precedent, textual interpretation, and social norms and traditions - is varied enough that plausible arguments can be made on both sides. On one hand, some judges - notably Justice Antonin Scalia - think the constitutional case for same-sex marriage obviously fails because nobody in 1868, when the 14th Amendment was ratified, dreamed that its equal protection clause applied to gay marriage. The right to marriage, which is deeply rooted in American history, was traditionally limited to opposite-sex couples. And there is no Supreme Court precedent subjecting laws that classify based on sexual orientation to the rigorous judicial scrutiny that has been applied to classifications based on race or sex. On the other hand, some judges have ruled that classifications based on sexual orientation should receive such scrutiny, as gay men and lesbians have long been victims of discrimination and still face social and political obstacles. They have also argued that marriage is a fundamental right that should not be narrowly circumscribed without good reason, and have found wanting the argument that marriage bears an inherent link to procreation. If constitutional doctrine will not determine the outcome of a Supreme Court case on gay marriage, what will? Politics. Today's justices divide five to four on virtually every important contemporary constitutional issue, including abortion, affirmative action, campaign finance, capital punishment and gun control. The court consists of two relatively stable four-person blocs of liberals and conservatives, with Anthony M. Kennedy hovering between them, usually casting the deciding vote. He has sided with the liberals in some cases involving abortion, the death penalty, and other civil liberties issues, but with the conservatives in most other cases. Gay marriage is likely to turn on Justice Kennedy's vote. What might it be? Justice Kennedy could, of course, follow a compromise path and rule that the Constitution mandates at least civil unions for same-sex couples. With two-thirds of Americans supporting civil unions, such a ruling would generate little political controversy. But were Justice Kennedy to rule on gay marriage itself, his decision would most likely depend on how he balanced two competing tendencies. As in decisions invalidating state sodomy laws and barring the death penalty for minors and the mentally disabled, Justice Kennedy's rulings often bolster dominant national norms and suppress dissident state practices. This approach would counsel restraint at a time when only six states and the District of Columbia permit gay marriage. On Election Day, same-sex marriage will be on the ballot in Maine, Maryland and Washington, and a constitutional amendment to ban it will be on the ballot in Minnesota. However, Justice Kennedy also explicitly embraces the idea of the Constitution as a living document whose meaning reflects evolving social mores. Partly on this basis, he wrote both of the court's pro-gay-rights decisions: the 1996 opinion in Romer v. Evans, which invalidated a provision in the Colorado Constitution barring state or local governments from enacting laws forbidding discrimination based on sexual orientation, and the 2003 opinion in Lawrence v. Texas, which ruled that sodomy laws violated the due process clause of the 14th Amendment. It's also notable that Justice Kennedy treats international norms as relevant to American constitutional interpretation. Gay marriage has recently been legalized in South Africa, Canada and many European nations, and is being actively debated in others. Justice Kennedy's opinions often suggest that he wants to be on the right side of history, which matters greatly here because the future of gay marriage in America is so clear. Support has increased from less than 25 percent in 1990 to roughly 35 percent in 2004 to over 50 percent today. At the current rate, a substantial majority of Americans will support gay marriage within the next dozen years. A Supreme Court ruling in favor of gay marriage would divide the nation roughly down the middle, much as the Court's ruling against racial segregation, in Brown v. Board of Education, did in 1954. Yet, within two decades, the Brown decision was almost universally revered. A decision protecting same-sex marriage would probably also soon become historic. Indeed, some lower courts have already recognized the significance of pro-gay-marriage rulings. The author of Massachusetts's pioneering gay marriage ruling compared it to that court's prohibition of slavery in the 1780s. In 2008, the California Supreme Court struck down a ban on same-sex marriage, proudly invoking its 1948 decision that invalidated a ban on interracial marriage. The California ruling was subsequently superseded by Proposition 8, a voter initiative that amended the State Constitution to ban same-sex marriage. Of course, predicting how Supreme Court justices will vote in particular cases is risky business. Who could possibly have forecast the convoluted path that Chief Justice John G. Roberts Jr. took to uphold the Affordable Care Act, President Obama's signature health care legislation, last term? But on the whole, it seems likely that when the case arises, Justice Kennedy will be tempted to write an opinion that would quickly become the Brown v. Board of the gay rights movement. Michael J. Klarman is a professor of law at Harvard and the author of "From the Closet to the Altar: Courts, Backlash, and the Struggle for Same-Sex Marriage."