The ratchet is a simple, ubiquitous, ancient bit of machinery. There's one in your bicycle wheel (it allows you to coast without pedaling), there's one in your watch (if you're the old-fashioned type and have a mechanical watch) and there's one in the jib sheet winches of your boat (if you're a yachtsman; but then in that case you probably aren't reading this book). What the ratchet does is permit rotation in one direction but not in the other. Here's a diagram:
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.
http://smithbowen.net/linfame/stopme/chapter02.html