Matthew Yglesias and Paul Krugman comment on the finding by political scientists Keith Poole and Howard Rosenthal that Congress is essentially one-dimensional in the post-civil rights era -- here's Yglesias:
We often think of a simple 2-dimensional models like the Nolan Chart in which people should be sorted along both a left-right axis about economics, and then along a second axis about social/cultural issues like gay rights. But as Krugman observes, Congress doesn’t work this way...
To offer some qualitative examples, Susan Collins and Olympia Snowe are pro-choice Republican Senators. But they’re also the two senators who seem like they might possibly vote for a national health care bill. Rather than representing some kind of ideal type of upscale northeasterner who’s socially liberal but economically conservative, they’re less conservative across-the-board than their colleagues from the South and the Mountains. Conversely, when you stroll down to Arkansas’ Democratic Senators, you don’t see cultural conservatives with populist economics, they’re just more conservative across-the-board than their coastal colleagues.
Indeed, this pattern of polarization along a single dimension of ideology is the historical norm, not the exception. The equilibrium was disrupted for decades by the issue of race and the South, but as Krugman notes, the convergence of the parties on basic civil rights and the realignment of the South has led to the decline of the importance of the so-called second dimension (essentially, race) in Congressional voting.
On the other hand, it's important to emphasize that Congressional unidimensionality is a product of institutions, not some sort of naturally occuring phenomenon. Politics is inherently multidimensional -- for instance, bill co-sponsorship patterns were found to have three to five dimensions. However, as UCSD's Gary Cox and Mat McCubbins emphasize, the majority party leadership in Congress blocks bills that would split the majority party from being considered on the floor, preventing alternate dimensions from being voted on. In addition, the combination of activist-dominated primaries and interest group fundraising make it difficult for any contemporary politician to diverge too much from the left-right ideological axis along an alternate dimension.