There's been lots of talk about what the GOP needs tonight for a big, huge win. One big factor is Republican turnout. Higher than normal - that would be great. Also, the independent vote should come in big for the GOP.
But neither is the essential ingredient. What the GOP needs at the end of the day is for voters to vote for Congress the same way they voted for president in 2004, which was a year of split intensity and one in which, incidentally, independent voters broke slightly to John Kerry.
Charlie Cook's report includes the "partisan voting index" for the races he has rated as competitive. It basically answers the question - in a 50-50 presidential year, how would these districts vote?
Looking at the Democratic seats only, these are the average partisan voting index's (PVI's) per competitive categories:
Likely Republican: R +6.75
Lean Republican: R +4.3
Toss-Up: R +3.3
Lean Democratic: D +1.9
LIkely Democratic: D +3.5
Really, if you drill it down to the essentials, this is what the GOP needs: it needs the Bush 2004 voters to come home to the congressional GOP. If this happens nationwide, you're going to see huge gains. Factor in that independents are swinging to the GOP in a big way (first time since 1994, incidentally), and you have a recipe for a history-making night.
This is the one critical factor the liberals overlooked in 2009 and 2010. They governed as though the average congressional district has a Democratic tilt, but it actually is tilted to the GOP. After all, George W. Bush lost the popular vote in 2000, but still won the popular vote in about 240 House districts. The Democrats had forgotten that their House majority really has depended on cross-over voters, people who supported Dole, Bush, and McCain but backed Democrats for Congress.