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.