| Over at Swing State Project, Shinigami wrote a great diary about Congressional districts where voters split their tickets.
The big picture is that Barack Obama carried 240 Congressional districts, of which 32 sent Republicans to the U.S. House. John McCain carried 195 Congressional districts, of which 49 sent a Democrat to the U.S. House.
McCain's advantage on this metric may surprise you, given how badly he was beaten in the electoral college, but Shinigami notes that
As has been the case since 1968, but with the exception of Bill Clinton in 1996, the GOP Presidential nominee, win or lose, has won more ticket-splitting districts than the Democratic Presidential nominee.
Click the link for the list of all the districts and some analysis of trends. The Obama/R districts are mostly in the midwest and the middle part of the east (Pennsylvania, New Jersey, New York, Delaware, Virginia), except for four in California, one in Washington, and two in Florida.
The McCain/D districts are mostly in the south, midwest and plains states, plus a few in the mountain west, two in New York and four in Pennsylvania.
Overall, just under 19 percent of House districts went for a presidential candidate from one party and a member of Congress from the other party. One of Iowa's five districts (IA-04) fell into this category.
Really, do click over to read the whole piece. It is worth your time.
UPDATE: In the comments, Pistachio thinks it's "crazy" that Republican Tom Latham was able to win by 21 points, even though Obama carried his district by 11 (actually, I think Obama's margin in the district was less than that). SECOND UPDATE: Thanks to Bleeding Heartland user Johannes for linking to this spreadsheet he compiled on the presidential voting in Iowa's Congressional districts. Obama won IA-04 by about 7.5 percent.
When the presidential votes by Congressional district have been tallied across the country (Swing State Project has these for the 2000 and 2004 elections and is working on compiling the same numbers for this year's election), it would be interesting to see which districts had the most ticket-splitting voters. LA-02 is a special case, because if not for the corruption allegations surrounding Democrat Bill Jefferson he surely would not have lost the runoff by three in a district Obama carried by 50 points. But there are other districts with disparities as large or larger than that in IA-04. For instance, Obama carried Delaware by 25 points, but Republican Mike Castle retained that state's at-large House seat by 23 points.