The American political system, since at least 1968, has been operating like a ratchet, and both parties -- Republicans and Democrats -- play crucial, mutually reinforcing roles in its operation.
The electoral ratchet permits movement only in the rightward direction. The Republican role is fairly clear; the Republicans apply the torque that rotates the thing rightward.
The Democrats' role is a little less obvious. The Democrats are the pawl. They don't resist the rightward movement -- they let it happen -- but whenever the rightward force slackens momentarily, for whatever reason, the Democrats click into place and keep the machine from rotating back to the left.
Here's how it works. In every election year, the Democrats come and tell us that the country has moved to the right, and so the Democratic Party has to move right too in the name of realism and electability. Gotta keep these right-wing madmen out of the White House, no matter what it takes.
(Actually, they don't say they're going to move to the right; they say they're going to move to the center. But of course it amounts to the same thing, if you're supposed to be left of center. It's the same direction of movement.)
So now the Democrats have moved to the "center." But of course this has the effect of shifting the "center" farther to the right.
Now, as a consequence, the Republicans suddenly don't seem so crazy anymore -- they're closer to the center, through no effort of their own, because the center has shifted closer to them. So they can move even further right, and still end up no farther from the "center" than they were four years ago.
