Both sides look to Justice Anthony M. Kennedy, a California Republican who is the author of the court's two strongest gay-rights opinions.
Advocates for what they call "marriage equality" believe that Kennedy, 76, would like to cap his career by writing a landmark opinion on gay marriage.
However, conservatives who oppose same-sex marriage note that Kennedy also has been a strong supporter of states' rights. They hope he will decide that federal judges should stand back -- at least for now -- and let the states and their voters resolve the fight over marriage.
It is not clear why the court voted to hear the cases or which way it is headed. It takes the votes of four justices to grant an appeal, but the court does not disclose these closed-door votes. It is not known whether the conservative justices voted to hear the Proposition 8 case to reverse the 9th Circuit, or whether the liberal justices wanted to rule on the right of gays and lesbians to marry.
Both cases also arrived at the court in an odd posture. California's top elected officials -- Gov. Jerry Brown and Atty. Gen. Kamala Harris -- refused to defend Proposition 8 in court. Instead, the defense was taken up by the sponsors of the proposition.
They are represented by Washington attorney Charles J. Cooper, a Reagan administration veteran who once hired as his deputy Samuel A. Alito Jr., who now sits on the high court.
In announcing they would hear the case, the justices said they also wanted the two sides to "argue the following question: Whether the petitioners have standing" to bring the appeal.
A similar issue is in play in the challenge to the Defense of Marriage Act. The Obama administration has refused to defend in court the part of the law denying equal tax, healthcare and pensions benefits to gay couples who are legally married. The House Republicans hired former U.S. Solicitor Gen. Paul D. Clement to defend the law. In Friday's order, the court said it wanted to hear arguments on whether the House Republicans had standing to defend a law in place of the executive branch.
In past cases Chief Justice John G. Roberts Jr. has been insistent on enforcing procedural rules and requiring that all cases have a proper plaintiff and a proper defendant.
The court's intervention came just one month after voters in three states -- Maine, Maryland and Washington -- approved gay marriages.
This brought the total to nine states having legalized same-sex marriages.
The justices are expected to hear arguments in the two sets of gay marriage cases in March and issue decisions by late June.
Times staff writer Jessica Garrison in Los Angeles contributed to this report.