Matthew Yglesias flags the latest Pew survey, which attempts to provide a political typology to describe the different ideological groups that reside in each party. Looking at the extent to which Democrats are divided on social issues and immigration, Yglesias notes the ideological asymmetry between Democrats and Republicans:
This is a big contrast from the structure of opinion among Republican loyalists, who are pretty uniformly rightwing. A Republican politician starts with a base of conservatives, and then needs to add some less conservative people. By contrast, while some Democrats have a base of “solid liberals” many don’t. And nationwide, the solid liberals aren’t just a minority of the country, they’re a minority of the Democratic Party base vote.
What's more, it's not just that the Republican Party contains more conservatives than the Democratic Party contains liberals but that "solid liberals" are less intense in their preferences than their conservative equivalences. If you compare Pew's typology groups on "Politics & Elections" and particularly, "support for compromise," you'll find that 70 percent of solid liberals like elected officials who "compromise with those they disagree with." By contrast, only 17 percent of "staunch conservatives" agree. The rest -- or at least, 79 percent -- prefer elected officials who stick to their positions.
Again, this contrast explains a lot about the current state of American politics. Republican politicians refuse to compromise because they are accountable to conservative voters who dislike compromise. Democratic politicians, on the other hand, are in the opposite position: Not only are they accountable to other groups besides liberals but liberals themselves prefer compromise.