for one thing, if 1968 precipitated a swing to the right, it was because of the swing to the left and much of the rhetoric of the left was adopted by the right--particularly about the state--in that swing, which is why the party that could be the left/liberal party in the US has no way left to articulate its argument--the far left (i.e. anything left of Bill Clinton) is off limits and anything else basically sounds like something some wing of the republicans might say. And in more material terms, if we adopt Doug's understanding of the early 1970s (which I think compelling in many regards), the initial swing to the left that caused the reactionary right was a fairly widespread popular movement. So if it's going to swing the other way, there would need to be people behind it.
In other words, there might be a pendulum here, but it ain't gonna swing itself.
s