One reason that cultural issues like abortion have been successful rallying points for Republicans is because such issues tend to beget an asymmetry of passion. While a majority of the country supports abortion rights under most circumstances, the average pro-lifer is probably more engaged by the issue than the average pro-choicer, thereby enabling the 45 percent to outweigh the 55 percent under certain conditions.
That may still be the case for abortion, where public opinion has been static for many years. But it may no longer be the case on gay rights. Just who is on what side of the 55/45 split depends on what question you're asking -- a majority of the public now supports civil unions, although not yet gay marriage. That's beside the point, though; what I think the Warren dust-up reveals is that the left is now willing to raise at least as much ruckus about the issue as the right. The left, of course, has always had its own moral compass, but it's now beginning to convert that into more focused, overtly political action. If John Kerry had won four years ago, and invited Warren or some analogous pastor to give his invocation, would there have been this much debate about it? It's hard to say for sure, but I don't think we would have heard very much about it at all. This all feels very recent, stemming from a renewed self-confidence on the part of the left, coupled in this particular instance with the aftermath of Proposition 8.
I say this as someone, by the way, who buys into the "Can't we all just get along?" side of the argument. There's a difference between feeling as though you have superior morals and feeling morally superior, and some of the discussion has veered toward the wrong side of that equation. Nevertheless, I think the passion aroused among the left on the issue has been fairly impressive, and is potentially fairly consequential.