The Supreme Court hearing on Proposition 8 — which will be argued tomorrow — has the potential to transform the national civil rights landscape for gay and lesbian Americans. More so than the case involving the Defense of Marriage Act, the Prop 8 ruling could ultimately result in marriage equality being the law of the land everywhere in the United States.

Gay rights advocates will be closely watching how Justice Anthony Kennedy frames his questions about what a broad SCOTUS ruling means for the states. If Kennedy appears uncomfortable with the implications of a broad ruling, that could — repeat, could — mean trouble. Kennedy is seen as the swing vote on the court, partly because he came down on the side of gay rights in Lawrence v. Texas, which struck down a state anti-sodomy law, and Romer v. Evans, which struck down a Colorado measure prohibiting localities from recognizing gays and lesbians as a protected class.

There are two ways SCOTUS could reach broad rulings with far reaching implications for gay rights. The first would be if the Court found a fundamental Constitutional right to marry, as it did in Loving v. Virginia, which struck down a state law forbidding interracial marriage, and other rulings. (This is what Ted Olson and David Boise, the lead lawyers for the plaintiffs, have argued.) The second would be if the Court found that laws that discriminate must pass a “heightened scrutiny” test — i.e. their rationale must be subjected to an extremely high standard — and that this scrutiny reveals that Prop 8 violates the Constitution’s equal protection clause. (This is what Obama’s solicitor general has argued.)

Either of these rulings would be historic, and could be used as a weapon to topple other state laws banning gay marriage as unconstitutional. So watch the discussions about the implications of such a ruling. If Kennedy asks whether ruling that way would be dictating on an issue that’s very sensitive to the states and short circuiting the political process, that could signal discomfort with going the broad route. On the other hand, Kennedy might also want to give Prop 8 opponents a chance to explain why such a ruling is legally defensible. Look for them to argue that this would not be an imposition of anything on the states, but rather that the states don’t have the authority to impose on gays who want to get married by depriving them of their constitutional rights.

It’s also possible that the Court could opt for a narrower ruling. As Richard Socarides explains, there are two options for the court which would essentially return marriage to California without implications for other state laws around the country. If questions seem heavily focused on procedure, it could hint at this direction.

If the Court does go bold, we could end up witnessing a remarkable and historic moment. As Jeffrey Toobin puts it, such a ruling could answer the following question…

Is there any circumstance in which the state can deny gay people benefits that are granted to straight citizens?

…with a resounding No. Either the Court would get to this point by affirming that laws that discriminate must be subjected to intense scrutiny, and when they are, it reveals them to be unconstitutional. Or, even more sweepingly, it could agree with the case made by the plaintiffs: that marriage is “central for all individuals’ liberty, privacy, spirituality, personal autonomy, sexuality, and dignity,” and therefore is a right protected by the Constitution. Either of these rulings would hasten the transformation of the country into a place where full equality for gay and lesbian Americans is the law of the land, everywhere — a transformation that now looks inevitable.

Greg Sargent writes The Plum Line blog, a reported opinion blog with a liberal slant -- what you might call “opinionated reporting” from the left.