Predicting what might happen tomorrow at the Supreme Court in Hollingsworth v. Perry is more difficult than with National Federation of Independent Business v. Sebelius (health care reform).
The Supreme Court would uphold the individual mandate or it would strike it down. The how and why mattered (I correctly predicted that the insurance mandate would be upheld as a tax), but the potential outcomes were few even after the cases were consolidated.
First a little history which will partially explain why there are so many possible outcomes.
Proposition 8 was a voter-passed initiative that defined marriage as a union of a man and a woman that was immediately subjected to lawsuits by same-sex couples and civil liberties advocates. The law was defended by ProtectMarriage.com and Campaign for California Families because the state of California believed the proposition was unconstitutional and refused to defend it themselves.
A federal district court in California struck down Prop 8 on June 16, 2010, finding that that marriage is not a matter of religion because of entanglement with government and the segregation of legal rights, that the state of California had no compelling interest (very important) in banning same-sex marriage.
The intervention of outside parties to defend a state law is what makes this case harder to predict than it should be. It's still not clear that the outside groups that have defended the law in court up until this point actually have the legal standing to do so. Normally the only parties who can participate are those who can sufficiently allege they are being harmed, or would be harmed by the law or action. In this case that means same-sex couples alleging discrimination have standing to sue because they are being banned from getting married, and the state of California in having its law struck down which potentially harms its stated interests in having that law in the first place. (One oddity being that the state didn't want anything to do with defending this law, so there are no stated interests for the state.)
You'll understand why standing matters when you learn of some of the potential outcomes. To put it simply, if defendants don't have standing to appeal then there's no chance that same-sex marriage bans will be struck down nationwide. The defendants must have standing (and then lose) in order to set that kind of precedent.
Outcome #1: The Supreme Court could uphold the ruling of the district court striking down Proposition 8 as unconstitutional under the due process and/or equal protection clauses (or through their own reasoning) and either ignore the question of standing entirely, or rule that the defendant-intervenors had standing.
That represents the best outcome for advocates of marriage equality. That holding would effectively strike down every state law and state constitutional amendment banning same-sex marriage in the country. Same-sex marriage would become legal in every state in the union and nothing could change that short of an amendment to the US Constitution, or a superseding ruling from the Supreme Court at a later date.
I won't attempt to predict how each justice will vote. I'm pretty sure that there are at least four votes for this outcome (Breyer, Kagan, Sotomayor, and Ginsburg) and Kennedy may be the swing vote. But Anthony Kennedy isn't someone I'd consider likely to go out on a limb if a decision can be seen as activist (even if it's not). Any conservative justice that feels more strongly about limited government than they do social conservatism may supply the fifth vote if Kennedy won't, like Chief Justice John Roberts did with health care reform.
If this is how it turns out, it's probably 5-4. I really don't think this lineup of justices has the courage to make this ruling though, even if a majority thinks it's the right thing to do.
Outcome #2: The district court ruling is upheld and all subsequent appeals are dismissed for lack of standing. The case will be remanded back to the district court for further proceedings which won't go anywhere because California still isn't interested in defending the law, and no outside parties will be allowed to step in as was done before.
Same-sex marriage will once again be legal in California, but it will be as if that ruling had never been appealed. All other bans in all other states will remain.
I can see it being 5-4 or maybe 6-3, leaning heavily towards 6-3. I think this is the most likely outcome, with outcome #1 coming within a few years.
Outcome #3: The district court ruling is overturned based on a technicality and remanded for retrial. The case will be argued before a new judge (original justice Vaughn Walker has retired) and Prop 8 will probably be struck down again. That ruling will be appealed to the district court, upheld, and appealed again to the Supreme Court putting us right back here in a couple of years.
The only way that happens is 5-4, and because the case will come right back, so I don't see this happening. I'd put the chances at less than 10%.
Outcome #4: The district court ruling is overturned on substance and same-sex marriage will remain banned in California.
I only see this happening 5-4 as well and since we expect the Defense of Marriage Act to be struck down, it'd be very hard to explain why DOMA is unconstitutional discrimination, but not Proposition 8. (If DOMA is struck down on a technicality like federal power overreach instead of due process and/or equal protection, that's a sure sign that outcome #1 isn't going to happen. If it gets struck down on due process or equal protection, outcome #1 is the favorite.)
Polls have shown a big swing in public opinion in California in support of marriage equality and it's likely that if Prop 8 is upheld, that it will be simply be overturned by voters with another referendum within a year or two.
Like in California if Prop 8 is upheld, that won't bar any state legislature or population from overturning their own bans in the future. It's possible -- even likely -- that most states that aren't strongly socially conservative will repeal their bans over the next 50 years.
Outcome #5: This is the same as #4 with the exception that the Supreme Court finds that while Prop 8 doesn't violation due process and/or equal protection, that reversing marriages that took place before Prop 8 was passed would violate the constitutional rights of those couples. It would preserve those marriages while paradoxically upholding the ban.
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I think #2 is the most likely outcome; the court upholds the striking down of Proposition 8 but finds that defendant-intervenors lacked standing to challenge that ruling. It'll be 6-3 or 7-2. Same-sex marriage is legalized in California, but otherwise it's status quo in the rest of the country.
Second most likely is #1, the court upholds the original district ruling that same-sex marriage bans violate the constitution's guarantee of due process and equal protection under the law. Same-sex marriage is instantly legalized nationwide.
That would be 5-4 all the way.
Don't be surprised, though, if we see DOMA struck down tomorrow but Hollingsworth v. Perry comes later this year.