Constructing Congressional Unidimensionality

I was musing yesterday about why it is that a single crude left-right axis does such a good job of modeling congressional behavior, when politics at least seems much more complicated than that. Brendan Nyhan brings a big piece of the answer:

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.

That makes sense. If Nancy Pelosi were constantly scheduling votes on trade policy, vote maps would look very different. But precisely because doing so would scramble her coalition, she wouldn’t ever do that.