Matt Yglesias puts up some solid polling data on how Americans self-identify as conservative, liberal, etc. Based upon this data, Matt concludes,
People who think of themselves as “conservative” are in a very real sense the “base” of the Republican Party. A politician who positions himself as to the right of Susan Collins but the left of Jim Jordan is doing what most self-identified Republicans want. But “very liberal” Democrats are a marginal block of people, and the median self-identified Democrat also self-identifies as a moderate. Liberals just aren’t the “base” of the Democratic Party in the same way that conservatives are the base of the GOP. Balanced news coverage is all well and good, but commitment to balance sometimes distracts people from the fact that the parties are very asymmetrical in their structure and composition.I think this explains why Democrats are able to triangulate, but not why the do so.
The explanation of why Democrats are willing to triangulate is simple pragmatism. Democratic leaders actually care about good public policy and want to use government policy to effect changes that they believe are in the best interest of the groups they serve.
Republicans on the other hand really don't care at all about public policy and think 'good government policy' or 'good government' are oxymorons. To over-simplify only a little, with the exception of defense spending and tax reductions, Republicans don't care if any bill ever passes out of Congress and generally believe the fewer, the better. Maybe put another way, with the exception of transferring money from government to corporate America -- and in particular the military industrial complex-- or tax reductions, Republicans don't care about policy. And when they do get behind policy we end up with Medicare Part D which is nothing more than a transfer of money from the treasury to Big Pharma.
Given this environment, if Dems don't triangulate nothing positive will ever get done.