The US Senate is dysfunctional because there are very few centrists.

The following chart, based on data from That's My Congress, shows how polarized the US Senate was on 22 selected votes in 2009.  Republicans Snowe (0) and Collins (-5) were slightly more progressive than Democrat Bayh (-7). All other Democrats and Independents were more progressive than those three, and all other Republicans were more conservative. 

That's how close we are to having zero ideological overlap between Senate Republicans and Senate Democrats/Independents. Note also how far the parties' centers of gravity are from the average of all Senators (-2.63). The average Republican (-49.55) is about 47 points from the center, and the average Democrat/Independent (28.65) is about 31 points from the center. Because the center is so unpopulated it is necessary for either party to skew legislation substantial ideological distances to pick up votes from the other side--especially to overcome a filibuster.  

If Democrats wanted to pass a measure supported by all of its most progressive members, it could get the 51st vote at a +12 ranking.  On the other hand, if it needed 60 votes to end a filibuster without Republican votes, it would have to skew to minus 7 to pick up the last Democrat. Republicans wanting to add 11 Democratic/Independent votes to their united caucus would have to skew the measure to plus 12 to find a 51st vote and to plus 20 (which is coincidentally majority leader Reid's ranking) to break a filibuster.

I'm quite sure the bimodal distribution of the Senate does not reflect the distribution of voters, which I expect is a normal bell-shaped distribution--high in the middle with two tails to the extremes. If I find the ambition and the data, I'll add a chart illustrating that. Further work could also include charting the House of Representatives, using issues selected by a conservative organization, and tracking back a few decades to see if there has been increasing polarization of Congress, as some pundits say. But what I'd rather do is access this work already done by others. Please leave a comment if you can steer me toward such work.

Update on Monday, March 15, 2010 at 05:06PM by Skeptic

This later post has a graph showing that in an October 2008 Time Magazine poll public opinions on selected polarizing issues were normally distributed, not bimodal like the Senate. 

Update on Sunday, May 1, 2011 at 01:22PM by Skeptic

Royce Carroll, et al. generated the following graph (and others) purporting to show that the polarization of the House and Senate has waxed and waned greatly since 1879.  In particular, the House is now considerably more polarized than the Senate, according to this work.

Unfortunately, I don't understand what data were used and even less about the methodology, but Nate Silver does and he discusses the study here. Silver uses data from the same study to show trends in the House over the last 100 years.