American public opinion on same-sex marriage has been steadily moving in the direction of support for marriage equality for some time, and recently some polls have shown a majority of the public in favor (see here for example). Politicians, however, have lagged the public on this issue, none more visibly than Barack Obama, who is famously "evolving" on the issue. One presumes that evolution will reach its higher stage some time after he gets re-elected, but you'd think that candidates running for lower offices might be a little more willing to come out in favor of marriage equality, particularly since it's so obvious that such a position will only become more popular over time. But as Jonathan Bernstein tells us, that doesn't seem to be happening, at least when it comes to Democratic Senate candidates. "The web sites of the 10 Democratic candidates running as challengers or for open seats show that very few of these candidates are eager to jump on this particular bandwagon. Only two—Elizabeth Warren in Massachusetts and Chris Murphy in Connecticut—trumpet their support for marriage equality and for repealing the Defense Of Marriage Act." This might seem surprising, but if we subject it to a cold-hearted Downsian risk-reward calculation, it actually makes plenty of sense.
Let's simplify things and assume that there are three basic types of voters: those who don't want any legal recognition of same-sex couples; those who favor civil unions (i.e., legal recognition that falls short of marriage); and those who favor full marriage equality. Let's say the first group is 30 percent of the electorate, the second group is 30 percent, and the third group is 40 percent (this is approximately what you get if you give poll respondents those three options). If you come out in favor of full equality, you've lost the anti-recognition group (at least on this issue), and you've got the pro-marriage group. As for the civil-unions group, you'll gain some of them and lose some of them; some will oppose a candidate who isn't right with them, but others will be OK with someone more progressive than they are. Hard to say how many, but if we split them right in half, the maximum proportion of the public you'll have with you on this issue is 55 percent (40 percent plus half of 30 percent).
Now let's say you come out in favor of civil unions. You'll still lose most of those who oppose any recognition for same-sex couples. But probably not all—let's say that one-sixth of that 30 percent, or 5 percent, is OK with where you are. And now, instead of having half of the pro-civil-union group with you, you've got all of them with you, at least on this. But how many of the pro-marriage group have you lost? Virtually none. In other words, by taking the civil-union position, you've got 75 percent of the electorate whom you haven't alienated.
The reason is that voters who favor marriage equality haven't yet demanded that politicians take their position in order to gain their support. If you are one of those voters, you've voted for plenty of Democratic politicians who are "evolving." You may not like this state of affairs, but you've come to expect it. There may be a voter out there somewhere who would rather stay home than vote for a Democrat who takes the civil-unions position, but if so, he doesn't have a lot of company.
This is a rather crude analysis that doesn't take into account the particular dynamics of each campaign, but it does demonstrate that once a candidate is sitting in the civil-unions position, there are approximately zero votes to be gained from moving from there to the pro-equality position.
All this assumes, of course, that the candidates have no actual principles and are making decisions solely on the basis of what will win them the most votes, which is obviously not the case. It also assumes that you can't win some respect (and donations) by demonstrating to voters that you are principled and courageous on this issue, which you probably can. But the larger point remains: at this point, Democratic politicians don't have a lot of votes to gain from coming out in favor of marriage equality. Eventually, we'll reach a tipping point where enough of them have embraced marriage equality that to not do so is what draws attention, makes base Democratic voters find you an unacceptable candidate, and costs you votes. But we're not quite there yet.