The New York Times has a good infographic on the repercussions of potential opinions. The Prop 8 case is Hollingsworth v. Perry. If the Court wanted to issue a sweeping ruling in Hollingsworth, they could strike down Prop 8 as in violation of the United States Constitution, which would legalize gay marriage everywhere. On the other side, the Court could overturn the lower court's decision and uphold Prop 8, maintaining the current marriage status quo.
The Obama Administration filed its own brief and Don Verrelli, its Solicitor General, will get time at the Court to argue. Interestingly, the Obama Administration makes the argument that Prop 8 should be struck down on the grounds that the distinction between civil unions with full legal benefits, which California recognizes, and gay marriage. This would destroy that distinction nationwide, and every state that gives full legal status to same-sex civil unions will de facto recognize gay marriage.
The Defense of Marriage Act case, Perry vs. Windsor, is much narrower in scope. Ruling that DoMA is unconstitutional would entitle legally-married gay partners to federal benefits in states where gay marriage is recognized - so gay couples would be legally eligible for things like military spousal benefits and social security benefits. The definition of marriage would largely remain a state-by-state issue.
Advocates for overturning DoMA have made strong arguments that federalism would commit the federal government to allowing and recognizing the definition of marriage as defined by each individual state. Ed Whelan at National Review argues that a brief supporting that theory is legal bunk:
Nick Rosenkranz over at the Volokh Conspiracy agrees with Whelan.
The brief falsely alleges that Congress has made an “unprecedented assertion that it has equal power to decide who can marry” (p. 26). But Congress, through DOMA, does not exercise any “power to decide who can marry.” It doesn’t nullify or preempt the state-law effect of any marriages authorized under state law. And, far from being unprecedented, it merely reaffirmed and made crystal clear what Congress has always meant by the term marriage in provisions of federal law.
The brief’s suggestion that DOMA “mean[s] that the national government can exercise all the same powers over family law that the States exercise” (p. 27 (emphasis in original)) is absurd. The brief repeatedly quotes the passage in the House of Representatives’ brief stating that “the federal government has the same latitude as the states to adopt its own definition of marriage for federal-law purposes” (emphasis added), but it evidently doesn’t understand what this passage means. Contrary to what the brief insinuates, the House is not claiming a general “police power” to regulate marriage. Rather, the House is merely making clear that the federal government’s authority in the realm of federal law includes the authority to define the term marriage in provisions of federal law.
A popular concern among both conservatives and progressives on the issue is, as the New York Times wrote, "the shadow of Roe v. Wade" that hangs over the proceedings. The idea is that a sweeping ruling in Hollingsworth would mobilize traditional marriage activists as never before and create a wedge issue that could divide Americans ideologically on a national level for decades in the future.
Even Justice Ruth Bader Ginsburg worried about this possibility:
Justice Ruth Bader Ginsburg, a liberal and a champion of women’s rights, has long harbored doubts about the [Roe v. Wade] ruling. “It’s not that the judgment was wrong, but it moved too far, too fast,” she said last year at Columbia Law School.
In Justice Ginsburg’s account, set out in public remarks and law review articles, the broad ruling in the abortion case froze activity in state legislatures, created venomous polarization and damaged the authority of the court.
“The legislatures all over the United States were moving on this question,” Justice Ginsburg said at Princeton in 2008. “The law was in a state of flux.”
“The Supreme Court’s decision was a perfect rallying point for people who disagreed with the notion that it should be a woman’s choice,” she added. “They could, instead of fighting in the trenches legislature by legislature, go after this decision by unelected judges.”
That general view is widely accepted across the political spectrum, and it might counsel caution at a moment when same-sex marriage is allowed in nine states and the District of Columbia and seems likely, judging from polls, to make further gains around the nation.
The likely June decision will definitely set the grounds for the ongoing gay marriage debate for years to come.