Republicans aren't "back," Democrats are "out." There's a difference. Obviously as the only other major party Republicans stand to gain quite a bit from Democratic misfortunes. However, the Republican "resurgence" will be short lived. Even if they retake Congress, any attempts to deliver on the "Tea Party agenda" and the inevitable failure of these attempts will send them further into the hole come 2012. Republicans are still running on the Reagan coalition + 1994! They need a new message if they want to hold onto power. If you strip away the politics and propaganda, Democratic ideas are more popular than Republican ideas. The Democrats have just done a piss-poor job of standing up for themselves as usual.
This has been my thinking. The only reason Republicans are doing well is because they're not Democrats, not on their own party's virtues.
