Alternatively the justices could reach a narrow ruling that strikes down just Prop 8 on the particulars and perhaps some other state bans on marriage equality -- while leaving other bans intact. One possible route for the court would be to say states may not take away same-sex marriage rights after they have granted them, as California did prior to the passage of Prop 8.
The defenders of Prop 8 -- ProtectMarriage.com, which spearheaded the original ballot initiative -- argue that marriage should be left to states. If the justices agree, Prop 8 (and other gay marriage bans) would be upheld and proponents of gay marriage would face a state-by-state battle to overturn existing bans.
The outcome is difficult to predict. Even though the national political tide is turning rapidly in favor of equality, and although swing Justice Anthony Kennedy has a gay rights streak, some court watchers doubt that the most conservative Supreme Court since the 1930s would leap ahead of most states on the issue and enshrine a right to marry. On the other hand, some lower court decisions point to a victory for same sex couples in California, if not other states as well. (The Obama administration recently championed a constitutional right for gays to marry.)
Alternatively, the justices could determine that the case lacks standing because California has refused to defend the law, and send it back to the lower courts to try again. This is less likely given that the Court agreed to take up two major gay marriage cases back to back.
The Tuesday oral arguments on Prop 8 will be followed Wednesday by arguments on the constitutionality of the Defense of Marriage Act, the 1996 law that prohibits federal recognition of gay marriage and denies benefits to legally married same sex couples.
Below is a chart detailing where each state currently stands on the issue of marriage equality.