I think, sadly from my view, that both parties will move towards social liberalism.
Aren't they always moving towards social liberalism (wouldn't a social conservative today be considered a liberal 50 years ago)?
Social issues are more "complex" now than they were fifty years ago, but I would wager that the average Republican motivated by social issues today is at the very least no more liberal than one fifty years ago. We're back to debating long settled precedents regarding social issues.
I think that the Democratic Party has quietly been shifting for the better part of two decades and has done so without causing great damage to the institution. It may have kept us as perennial underdogs for some time, but that time is coming to an end as the demographics spell doom for one party, and it's not us. The Democratic Party has moved closer to the center than it was in past decades (I've never understood people saying Democrats are more liberal than they were when it comes to economic issues). Both parties have become more conservative on economics, with the main difference now being on social issues. Democrats champion old Republican economic policies while the new Republicans champion something that can only be laughed at by Democrats and any old school Republican.
The Republican Party, on the other hand, is heading face-first into something that they cannot retract from until the party finally implodes. Out of the ruins of the social conservative movement will come the three components: Corporatists, Libertarians and Evangelicals. I feel that the Evangelicals will find themselves without a home in the coming years, as I'm sure the Libertarians and Corporatists can work something out (social liberalism with corporate capitalism that masquerades as free enterprise).
Well then 100 years or something, a significant length of time ago. But you're entirely correct on how the parties have been shifting, or at least the Democrats. I could see the Libertarians abandoning the Republicans quite soon to either make a significant LP or join the Democrats (people a la Schweitzer, Tester, Webb, and Paul Hackett), with a Corporationist-Evangelical alliance (you'll be conservative economically, we'll be conservative socially). Hey, I vote Democrat locally (for my State Rep./State Senator/Governor/Mayor, and usually do so for House/Senate when there's no LP candidate or the Republican's a social conservative). When Ralph Nader's backing Ron Paul, this is entirely possible.