Yglesias encapsultates one of the things I worry about the most (at least in terms of politics):
I think it’s very possible that Democrats could “gain so much power” that they implement at least some of their “crazy plans” and that the people, rather than revolting, will just turn their attention to other issues. For example, many Americans feels anxiety about their health insurance status. And the majority of these people vote for Democrats. But if Democrats deliver a health care reform plan that assuages those fears, those voters may start voting more on their hatred of abortion or love of torture and bring Republicans back into power.
It doesn't always work this way, but it's a very real possibility. There are other ingredients at play, of course, that could shake up a scenario like this. But unless we're experiencing a major party shift here -- the first since the Whigs vanished -- the Republicans will be back. It's more a matter of how, how soon, to what extent and which branch.