Why The Supreme Court is Likely to Rule for Gay Marriage Kris Perry and Sandy Stier, plaintiffs in a lawsuit challenging California's same-sex marriage ban, pose in San Francisco on March 21, 2013. The couple, together for 13 years, will travel to Washington as the US Supreme Court considers their case. The Supreme Court hears arguments tomorrow in two historic cases about whether same-sex couples have the right to marry. It is always difficult to predict Supreme Court rulings, but there is good reason to expect some kind of victory for marriage equality. The main reason: Justice Anthony Kennedy, the man who is likely to cast the deciding vote. The court is considering challenges to the Defense of Marriage Act, which bars the federal government from recognizing same-sex marriages, and Proposition 8, the California ballot initiative that bans same-sex marriage in that state. These challenges are historic: though state and federal courts from Alaska to New Jersey have considered same-sex marriage, the Supreme Court has never heard a case about it. MORE: Why Republicans Are Saying "I Do" To Gay Marriage The Supreme Court is known for its sharp partisan divide. The four-justice liberal bloc is likely to be sympathetic to gay marriage, while the four-justice conservative camp is likely to be hostile - though how Chief Justice John Roberts will come out is far from certain. In the middle is the Court's usual swing justice, Justice Kennedy, who has - surprisingly - been the court's most steadfast supporter of gay rights. A Reagan appointee, Justice Kennedy is no liberal, as he has shown on issues from affirmative action to corporate campaign spending. But he has repeatedly sided with gay litigants before the court. In 1996, early in the gay rights legal revolution, he wrote the majority opinion in Romer v. Evans, striking down a Colorado constitutional amendment that prevented localities from passing laws protecting gay people from discrimination. In 2003, he wrote the landmark ruling Lawrence v. Texas, which struck down Texas's law against gay sex. MORE: What Will Justice Kennedy Do? It is not clear why Justice Kennedy - who has not been a particular friend of racial minorities in civil rights cases - has been so sympathetic to gay rights. One factor could be that, as a law professor told the Los Angeles Times, he is a "California establishment Republican" who has traveled "in circles where he has met and likes lots of gay people." A new PewResearch poll found that the biggest factor in changing people's minds in favor of gay marriage is knowing a gay person. Or it could be other factors: people have all sorts of reasons for the beliefs they hold. What matters is that in Justice Kennedy's case, the sympathy for equal rights for gay people seems both sincere and deeply held. In his 2003 opinion striking down Texas's sodomy law, Justice Kennedy not only said that the court's 1986 ruling upholding a similar Georgia law was wrong - he insisted that its "continuance as a precedent demeans the lives of homosexual persons." As they say on Wall Street, past performance does not guarantee future results, but it would be surprising based on Justice Kennedy's rulings so far if he did not side in some way with the supporters of gay marriage. If he is joined, as expected, by the court's four liberals - Justice Ruth Bader Ginsburg, Justice Stephen Breyer, Justice Elena Kagan, and Justice Sonia Sotomayor - there will be five votes on the court sympathetic to the pro-gay marriage side. Those five justices could well unite to hand down a sweeping, Brown v. Board of Education-style ruling that the equal protection clause requires all 50 states and the federal government to recognize same-sex marriage. But there are many reasons that, owing to the specifics of the cases, the justices might do less. One option for the court in the Proposition 8 case is to say the parties do not have legal "standing" - and that they should not decide the case at all. That would leave in place a lower-court ruling allowing gay marriage in California. Even some supporters of gay rights suggest there might be good reasons for the court to move slowly. They argue that gay marriage is doing well right now in the political process - and that it will eventually be adopted nationwide through legislatures and referenda. And they caution that having the Supreme Court order it could create the sort of backlash that emerged in 1973, after Roe v. Wade struck down abortion restrictions nationwide. In the end, the law is what five justices say it is - and based on Justice Kennedy's track record, there may be the votes for a far-reaching decision. Justice Kennedy has not been particularly tentative in his gay rights rulings in the past- he has written two landmark decisions and this could be his third. If Justice Kennedy is disturbed when the law "demeans the lives of homosexual persons," he may be eager to provide the deciding vote to usher in marriage equality.