I love XKCD. Here, he has graphed the DW-Nominate scores for both chambers of our Congress. Check out the full size version: http://xkcd.com/1127/large/
The DW-NOMINATE data set is a treasure, and I’d been planning on doing something similar to this graph to follow my Supreme Court post. But since he’s already done all the hard work, let me just make a few observations:
Polarization
Take careful note of the way the parties have changed since Nixon. Not only have the conservatives and liberals fully sorted themselves into the two parties, the centrists of each party (shown by the lighter color) have been displaced by the extremes (the darker colors). This is especially apparent among the House Republicans, where that deep dark red took over swiftly beginning in the 1990s.
There can be no doubt that this trend is a failing of our republic. It’s a political failure, instigated intentionally by folks like Pat Buchanan who started as an advisor to Nixon. It’s a media failure, brought on by the way technology has replaced a unified national discussion with a fractured ideological one. And it’s an institutional failure, seen in the corrupted redistricting of House seats and the incentives to abuse every one of the numerous veto points in our legislative process.
The Historical Switch of Left and Right
On a historical note, let me say that I do not agree with DW-NOMINATE’s classification of right and left in the first three party systems. I, for one, strongly associate myself with the Washington Administration, not the anti-federalists; with Hamilton, not Jefferson; with Henry Clay and Daniel Webster, not Andrew Jackson; with Lincoln; and with the Progressives of Teddy Roosevelt, not the Populists of William Jennings Bryan. In each of these cases, I think that yesterday’s left has become today’s right, and yesterday’s right the left.
But maybe it’s just that my tribal affiliation with the North is stronger than my partisan affiliation? Does anyone else see the left/right switch in the same way I do?
PolityMaking 
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Interesting idea. I’m exactly the opposite – Jefferson/Bourbon Democrat up through Grover Cleveland (with brief sojourns as conservative Whigs and Republicans under Jackson and Lincoln, respectively), conservative, anti-Bryan, Democratic-leaning swing voter through 1932, then firm Republican. I have to commend DW-Nominate for not setting an arbitrary date for a 180-shift, though. It’s a purely statistical technique, and it does a marvelous job of defining that slippery quintessence “Republicanism” at a consistent +1.0. That’s all we can really ask for, without going far beyond two dimensions.
As to the broader point, it’s definitely true as a sociological trend (Yankees, Blacks, civil servants, and the liberal gentry vs. Southerners, alienated rural/exurban yeomen, and the economic sectors that are most hurt by big government, from antebellum tariffs to modern regulation), and to the extent culture is the shadow of sociology, it’s also a cultural trend. But as much as it’s as simple as that, on the flip-side, it’s also a lot more complicated than that. For instance, you can see definite echoes of both the Fire-Eaters and the Radical abolitionists in the Tea Party (look at Ron Paul – combines the both). And look at the Religious Right. “Rum, Rebellion, and Romanism” fundamentalists joining hand-in-hand with Catholic traditionalists against the secular liberalism that once ran through the heart of both parties.
Culture is a beautiful thing. Organic trends set the stage, political expediency molds the coalitions, and, here’s the thing, people actually convince themselves that their accidental vote-buddies are the proverbial “us” in the great cosmic “us vs. them.” How much of the Southern realignment was simple regional animus against a port-listing Northeast? How much was cynical conservative race-baiting? How much was the re-emergence of the old Scotch-Irish spirit? How much was the industrialization of the Sunbelt? And how much of it was a genuine shift to the libertarian right, catalyzed by years of conservative Northern migrants with whom they shared a common disgust for liberal overreach?
And to today’s liberals: how much of your ideology is simple patrician disgust at the unwashed who define the popular image of the Tea Party? How much is genuine philosophical conviction? And how much are the two intertwined? Be honest now.