As in life, it's complicated but take note:
The Democrats will win the turnout game and add over 4% to their vote totals. So every tossup race that is within 4 percentage points goes to the Democrats. Any race that isn't already gone well over 50% for the Republican will go to the Democrat. The only exception is possibly Feingold in Wisconsin, where he will get a hidden sympathy vote for having run a clean campaign.
The idea is that this was a Republican wave year. All year long. If you're a Democrat with a chance after all of this, you got re-invigorated and go on to victory. Think of it as an NBA game where you're getting beat badly the whole game, but you never get "put away". Then in the fourth quarter you're down only a few points. If it was 8 or 10 you'd probably give up, but less than 4, you know how to win, because you've done it before. Mark it down.
Democrats win the close races, Republicans win the blowouts. That's how they will win the overall vote but lose both houses of Congress. The Triage strategy works. It doesn't really matter if you lose by 3 or 4 or get blown out. Hello Jack Conway!!! He was going to lose either way. But now, it will look non-competitive. Whereas, Harry Reid will win closely, Patty Murray, Manchin, etc. Net Net the Republicans get more votes combined but lose.
Watch out for the conspiracy theories about how Democrats are "outperforming" their polls.
In reality, it's that Democrats, especially in the House of Representatives are going to get most of the undecideds and a lot of "unlikely" voters. The people who are registered and discounted in all the polling because basically they don't care.
I hope the Tea Partiers can handle reality. Here goes:
Democrats 56 Republicans 44
Feingold wins Wisconsin with 53%
McAdams win Alaska with 35%
Manchin wins West Virginia with 52%
Murray wins Washington with 53%
Sestak wins Penn. with 51%
Boxer wins Cal. with 54%
Blumenthal wins Conn. with 55%
Reid wins Nevada with 51%