In a recent article in the Weekly Standard, Jay Cost notes that the House is more often Republican than the Senate:

... the Republican party has won control of the House of Representatives in 9 of the last 11 elections and the Senate in 6 of the last 11.

Cost’s explanation, although probably valid, is unnecessarily complicated.  It seems obvious that the most prominent feature of all recent elections has been the split between urban and rural votes.  We are not a nation of red and blue states; we are (except for New England and New Mexico) a red nation with intense blue polka dots, as shown here:

We look like someone with a bad case of acne.

Thus, the House elections, which reflect the real population differences, tend to be won by Republicans.  In contrast, in the Senate elections, the overwhelmingly blue urban districts of several states outweigh the red rural ones, and the Democrats win the whole state.

In 2004, Washington would have gone to Bush of it had not been for the 2-to-1 Kerry vote in the Seattle area.  Like emotionally incompatible but hopelessly joined Siamese twins, Seattle and rural Washington want to go in different directions.  In the ensuing tug-of-war, Seattle usually wins.  The situation is similar in other states such as Chicago vs. Illinois, Detroit vs. Michigan, New York vs. New York – it sounds like a divorce court roster.

I propose that we grant those divorces.  Germany solved this problem long ago by giving former “Hanseatic” cities, like Hamburg and Bremen, the status of states.  I propose that we grant statehood to New York, Chicago, Los Angeles, Detroit, etc. and add them to the Union.  The remaining states, predominantly rural, would then go their own conservative ways.

I can just see Seattle and Washington, hand in hand, saying, "We're better friends now than when we were married."