Yes, we've been down this road before. But let's get real: everyone in the country knows about the Massachusetts Reality Warp (except maybe the people in this state, many of whom don't seem to understand just how weird a place this is). So when MA legalized gay marriage, it raised quite a hubbub, but in a certain sense it wasn't a threat -- most people could take say, "Oh, that's just that strange liberal state", and figure that it wouldn't happen anywhere else.
But this time it's California. Still pretty liberal, but quite a bit more divided and representative of the country -- this is the state that brought us Ronald Reagan, after all. It's the biggest and richest state in the country, and it tends to be a long-term bellwether. So when *they* legalize it, both sides know that the ball is gaining momentum. I think most people are going to have the sense -- the quite accurate sense, I should add -- that if the gay-marriage movement isn't stopped within the next five years, it'll pretty much be unstoppable, since the demographic trends are on its side.
So the religious right has only one tactic that makes any real sense: put pressure on McCain to change his stance, and support a constitutional amendment banning gay marriage. The excuse they will use is that it's fighting "activist judges", who are perverting the country's morals. Their claim will be that, by turning this Presidential election into a referendum about holding a referendum, they will be able to rally the right once more, and take the White House.
What will McCain do, if the conservatives push this? He's on the record as being against an amendment -- indeed, his public position is much like that of both Democrats, and if they all keep to their positions the issue will be relatively muted. (Which, mind, is the best likely outcome. We *will* win this, so long as we play the long game, winning victories state by state. The only chance of real defeat is if the conservatives can rally strongly enough to get and win a constitutional amendment now, which would likely set the whole thing back by decades.) Oh, sure -- there will be the battle over who gets to appoint the Supreme Court, which is somewhat relevant. But that just doesn't inspire the kind of fervor that an amendment could, especially since the conservatives *already* control the Court.
But McCain isn't as consistent in his views as he would like to pretend, and he *is* a very serious conservative -- of the three candidates, he's the only one who I suspect is genuinely against gay marriage in principle. (As opposed to the Democrats, who are probably unsupportive mainly as a matter of sad but sensible political tactics.) I would say it's entirely plausible that he will find a way to amend his public stance in order to look more conservative -- he's done it before. Indeed, the only reason I consider it fairly likely that he won't is that he's already the Republican candidate, so he's tacking a bit more towards the center now.
Still, this election is going to be all about rallying the troops. McCain doesn't have particularly deep support from the current core constituencies of the Republican Party, and his greatest danger is that they simply won't vote. Yes, he's got a lot of support from the center -- but much of that is predicated upon a persistent misimpression that he's a social moderate, and I think it's unlikely that that is going to survive the next few months. So he has a nasty political calculation to make, about whether he needs to do *something* that will not only make him look credible to the social conservatives, but make them passionate about voting for him.
Will he change his stance? I dunno. I hope not for several reasons, not least that it would damage my already-wounded respect for the man. I would be much happier to see him stick to his guns: being a man of principle is about actions when the stakes are high. If he does switch to supporting an amendment, everyone should prepare for what will likely turn into the real ground battle over gay marriage -- given the timing and circumstances, I suspect that this would be the biggest and most important fight over the issue.
So let's hope for status quo, but be ready for possible rapid escalation of the issue...