If Democrats and Republicans can agree on one thing, it's that they both want "change." Barack Obama called for it in 2008, and Republicans are calling for it now. What that "change" means is of course up for debate, but one thing's for sure: 2010 will be the third straight change election.
Chuck Todd made this observation recently and noted that "in 2006, Democrats won control of Congress. In 2008, Democrats won the White House. And in 2010, Republicans appear poised to take back control of Congress." Chuck added that only three times since World War I have 20 or more seats flipped in three straight elections -- after World War I, during the Great Depression, and after World War II.
The map below shows fairly steady voting patterns from 1976-2000 through most of the nation, with the Mississippi River Valley and the Ohio River Valley standing out as the nation's preeminent battlegrounds. The map is unfortunately somewhat outdated, but as recently as 2006, Stu Rothenberg wrote that "the Ohio River has become a focal point of American politics recently, and that isn't likely to change this year."