The above graph is courtesy of “John Coleman”:http://users.polisci.wisc.edu/coleman/. He writes via email:
bq. In this graph you have, since 1950, the midterm seat change for the president’s party in the US house and the losses in the lower houses in the state. They track well, including the unusual years of 1998 and 2002. Perhaps most notable is that the tracking occurs both in lean and not so lean years for the president’s party, suggesting a reasonably well integrated party system in which voters hold a party accountable up and down the federal electoral ladder.