Well, the honest truth is, middle America still feels more comfortable in terms of security/economy with the Republicans in charge. The Democrats have done nothing in terms of rectifying their image problems in that regard.
So, unless the [R] incumbent is some sort of creep (Santorum, DeWine, Allen) people will probably keep the status quo. The economy is in decent shape around much of the country and the Iraq war is a mess, but lacks the Vietnam level of casualities to cause America great concern.
I saw the recent Pew study that showed many of the remaining "undecided's" are Republican voters. So expect them to close
gap in alot of races that were leaning Democrat. Just depends on each individual race.