I'd like to respond to Devin's comment that gay rights and not abortion is the defining social issue of the day. First, I should say that Devin and I have talked about this at some length and so some of what I write here will likely be unconsciously stolen from something he said.
I wonder if it's really at all possible to pick a "defining" social issue. Certainly, the typical combo is abortion and gay marriage. But guns, affirmative action, school prayer (and the fight over the secularization of society in general), immigration (and nativism) have each in recent years played the sort of role that a social issue does. Perhaps we're off on the wrong foot when we try to pin down one "defining" issue, by which I believe we mean an issue that seems to determine one's general social attitudes. While it might be interesting and even useful to track individual issues and their salience over time, it's not clear to me that Americans branch "social issues" into separate fights with specific contexts. On the other hand, I get the impression that social attitudes tend to all run together. "Social conservatives" would be those who for whatever reason feel that something fundamental and worth hanging on to in American culture is being annihilated by feminism, gays, civil rights, abortion, secularism, or something. "Social liberals" find these views, and the outrage that accompanies them, to be totally incomprehensible. "Social moderates" maybe sympathize with both camps, and simply don't get all worked up about the fights we're talking about.
That's not a new postulation. Since the 1960s ended (around 1974), we've been told that every election was a referendum on the 60s. Even Obama has been known to use this rhetoric, arguing that most of us are really social moderates, while our conversation on social issues has lately been driven by old 1960s activists for either side.
Which is not to say that such a formulation is correct. And even if it is, that does not eliminate the possibility of a "defining social issue". However we mark off the camps, it's possible that we'll find one issue on which the two sides are perfected divided. Devin made the point to me that abortion is certainly not that issue, given the large number of prominent pro-life Democrats and pro-choice Republicans (even among party elites and activists). But if abortion is not the issue, I'm not yet convinced that any issue is.
Besides, finding a "defining social issue" doesn't help us to interpret this poll. We have no idea what each respondent thought was meant by social conservative, moderate, and liberal (nor do we really have any clue on the economic issues).
Perhaps, then, what this poll does isn't to break voters into ideological groups based on actual positions, but to break voters into groups based on their comfort with certain labels. Knowing that a voters considers them self a social liberal rather than a moderate does tell us something about them. But it doesn't tell us the same thing that we would get from asking about individual social issues and concrete positions for them.
Anyway, this kind of stuff just screams for the Pew Survey or something.