Let's start with the House and Senate parties. House Republicans have veered dramatically to the right in recent years, driven in part by the aggressively right-wing views, suspicion of leadership, and iconoclasm of many in the recent freshman classes. I have written before about the significant differences between House and Senate Republican approaches, reflected especially sharply in the "fiscal-cliff" vote several months ago. The last-minute deal created by Joe Biden and Mitch McConnell, which allowed taxes on the wealthy to return to pre-Bush levels, got 89 votes in the Senate, including all but five Republicans. But almost two thirds of House Republicans voted nay.

In part through redistricting, in part because of the "big sort," with Americans increasingly congregating in areas where they are surrounded by like-minded individuals, more and more House Republicans represent homogenous districts that tilt Republican. In the modern media age, their constituents get information from the same talk-radio and cable-television sources, and these modern media reinforcements tilt sharply right. For many of the lawmakers, they themselves believe what their constituents believe. For others, the consequences of voting in a different way are clear.

Although the Senate Republican Conference has some extreme ideologues, it has a much smaller share than its House counterpart. Why? Lots of states have significant heterogeneity, in partisanship, ideology, race, and ethnicity; statewide elections in more-competitive states create plenty of incentives to seek common ground or to appeal to a wider range of voters. True, the obduracy of Senate Republicans under McConnell -- the willingness to sabotage anything that might benefit President Obama and to use filibusters in unprecedented ways -- does not make the Senate GOP a bedrock of compromise and problem-solving zeal. But on issues ranging from immigration to farm policy to deficits and debt, pragmatism is far more prevalent in the Senate Republican Party than it is in the House.

What about the presidential party? It should be more like the Senate, but the nature both of the ideological media wind machine and of the primary and caucus process have moved it sharply to the right as well. The shrill, over-the-top op-ed written by presidential hopeful Bobby Jindal recently in Politico is telling. So is Marco Rubio's willingness to consider carrying the water of antiabortion forces to promote a Senate bill to ban abortions after 20 weeks.

Now come the other two parties. The House votes on the fiscal cliff, aid for victims of Hurricane Sandy, and the Violence Against Women Act showed a fascinating regional pattern. Of the 151 GOP nay votes on the fiscal cliff, 82 came from Deep South Republicans -- 87 percent of all the Southerners, compared with 47 percent of the non-Southerners. (Border South Republicans are a lot like the deep Southerners.) Of the 67 nay votes on Sandy, 31 came from Southerners. And of the 138 Republican nay votes on VAWA, 79 came from Southerners. Southern Republicans as a whole, not just those in the House, reflect a distinctly different political framework, culture, and attitude than others. If one looks at the loony statements made by political figures over the past couple of years, some may come from the likes of Michele Bachmann and Steve King, but more come from Southern GOPers like Rep. Louie Gohmert of Texas, Rep. Paul Broun of Georgia, Sen. Ted Cruz of Texas, and assorted state legislators and party chairs.