I try to approach this by asking, "What are the drivers that will determine the future?" The simple answer is "Ideas - always ideas."
But ideas don't march in like they had legs. They come in different 'containers,' if you will.
In the short-term we see the swings of the federal elections that appear to be pendulum-like where bad times will drive out the party in power, particularly the democrats, and bring back the republicans... temporarily. This was seen with Carter being replaced with Reagan. But when the Republicans forget their campaign promises (remember "Contract with America"?), or experience a loss of support by their base (usually because they being to behave just like Democrats), or it seems like good times are back, then the pendulum shifts and our tolerance for altrusitically disguised progressive ideas bring back the left.
These factors seem to account for changes that cover one or two voting cycles. I see this as movement nearly always falling within the Overton Window and effected by the ideas the people have about the current economic situations and the intensity of their desire for change in Washington. If things end up pushing the edge of the Overton Window one way or the other, it just makes the pendulum swing back faster next time.
But that cycle seems to live within a larger cycle which I believe is driven by both generational change and by the tendency for big government to grow bigger. This larger cycle involves the movement of the Overton window itself. For many generations now, each of the younger generations are more to the left then the ones before. (We also have more and more libertarians and they are coming more and more into the public arena - but my sense it that the left is growing faster and finding more influence). These students, who are mostly to left, come out of college, go to work, and vote, and start moving into the careers that will end up effecting people's perceptions (journalists, talking heads, the next set of professors, etc.) This driver is the one that sets the longer trend - the trend that has been driven by our culture moving strongly to the left in the universities. The large demographic bulge in our population - the Baby Boomers - is an historic anomoly and one that will cease to have an effect on the culture in a decade or two and that will result in a change of equilibrium and things will move much faster to left - all else remaining the same.
I don't see anything in the near to medium term that would change this trend unless for some reason we have a major crisis, probably economic, AND the population recognizes that the left is lying when they say it wasn't their fault. More likely is that we will have a major crisis anywhere from next year to 20 years down the road, and the media will go along with Progressives in avoiding any blame. That's really bad because it will be justification for making larger transformations - emergency actions that never go away.
On a yet longer scale there will come a time when Objectivism will become the driver because in the long run, better ideas do win out. But that might be a long, long time from now.
What I see now, with such strong factionalism, and the drive towards PC thinking and expression, and our increasingly failed educational system is that we will move in the direction of another dark age. Every year more and more is NOT passed on to the next generation and they then don't have those things to pass them on when it is their turn to teach - and those things are lost like they fell of a clif and disappeared. But on the other hand, technology is almost like a self-sustaining explosion - not linear in growth, but exponential. These are two powerful drivers in our culture: Technology (and the wealth it creates) versus the dumbing down of the culture in so many other areas combined with the harmful effects of progressivism.
Anyhow, that's how I see it.