Liberals have been lamenting that the Democrats have surrendered the war of ideas and allowed Republicans to frame the budget debate entirely on their own terms, including the borderline-crackpot notion that cutting the deficit will reduce unemployment immediately. Why are Democrats doing this? Well, one reason is that the Democratic base wants its leaders to compromise, and the Republican base doesn't:

Over all, 55 percent of Americans, including 53 percent of independents and 69 percent of Democrats, want lawmakers whose views they agree with to compromise. But 50 percent of Republicans, including 56 percent of conservative Republicans, want lawmakers who share their views to stand by their principles, even if that means the government will shut down. Among Republicans and Republicans-leaning independents who agree with the Tea Party, 68 percent want lawmakers who share their views to stand on principle.

Most people have the default assumption that the two parties are essentially mirror images of each other. But there are a lot of asymmetries between the Democratic and Republican parties that result in non-parallel behavior. The Republicans have a fairly unified economic base consisting of business and high-income individuals, whereas Democrats balance between business, labor, and environmental groups. The Republican Party reflects the ideology of movement conservatism, while the Democratic Party is a balance between progressives and moderates.

The upshot is that the Democratic Party is far more dependent upon the votes of moderates, who think of themselves in non-ideological terms and want their leaders to compromise and act pragmatically. The reason you see greater levels of partisan discipline and simple will to power in the GOP is that it has a coherent voting base willing to support aggressive, partisan behavior and Democrats don't. This isn't to say Democrats are always wimps, but wimpiness is much more of a default setting for Democrats.