Andrew Gelman has an interesting chart up at FiveThirtyEight showing the relative levels of support for gay marriage in the 50 states, and how they have shifted over the last decade-and-a-half or so. Looking at the numbers in the context of competitive races this cycle gives some insight into the extent to which this week's decision by a federal judge overturning California's ban on same sex marriage will play in the midterm elections.
I think it's safe to say that inasmuch as the ruling will provide a boost for either party, it will be for the Republicans, giving them another sharp stick with which to prod their angry base voters. (Keep in mind too that President Obama opposes gay marriage.) Nate Silver made this argument Wednesday at FiveThirtyEight, pointing out that while the Tea Party has avoided social issues, the establishment has every reason to focus on it.
My best guess is that the Tea Party will largely continue to shirk the issue, but that the Republican Establishment will be fairly happy to engage it. The real battle, however, may come in 2012, when the Supreme Court could be about ready to take up the case. The leading indicator may be the reactions of the major Presidential hopefuls.
Regarding the 2010 elections, the question is where this is a powerful issue. First a bit of context: the high water mark for support of gay marriage is in Massachusetts, where 55 percent approve. It's one of only six states with majority support for same sex marriage. So even support in the 40s is, in this spectrum, relatively favorable (or relatively not unfavorable). Or to put it another way, one would expect the GOP to be able to get greatest traction for the issue the farther a state moves from 50 percent support.
The Cook Political Report currently lists 13 Senate races (eight seats currently held by Democrats and five now in GOP hands) as being toss-ups. Of those, seven are in states with support for gay marriage that tops 40 percent: Democratic incumbents Barbara Boxer in California (right around 50 percent), Patty Murray of Washington (where support is a bit under 50 percent), Michael Bennet of Colorado (where support looks to be a bit under 50 percent), and Harry Reid of Nevada where support is a bit under 45 percent (up from about 25 percent in the mid-1990s), and the seats being vacated by retiring Sens. Judd Gregg, Republican of New Hampshire (where support is just under 50 percent), Democrat Arlen Specter of Pennsylvania (around 45 percent support), and Democrat Roland Burris of Illinois (between 40 and 45 percent).
Note that virtually all of those seats are Democratically held. When you dip under 40 percent support, the states get a lot redder (no great surprise there), and a lot more open. The only incumbents are Democrats, suddenly endangered Russ Feingold of Wisconsin (under 40 percent support for gay marriage) and should-be-updating-her-resume Blanche Lincoln of Arkansas (under 25 percent support). The four other seats are all being vacated by GOPers: George Voinovich in Ohio, (under 40 percent), George LeMieux in Florida (over 35 percent), Kit Bond in Missouri (under 30 percent), and Jim Bunning of Kentucky (a bit over 25 percent).
(Kentucky is an interesting case, as GOP nominee Rand Paul is ostensibly a libertarian but hews to a social conservative, non-libertarian line when it comes to gay marriage.)
So the take-away here for Democrats is probably that the decision was not helpful for 2010 races, but given the number of states in play that are fairly moderate on gay marriage, it could have been worse.