George Bush and the Tea Party Movement
Bush is responsible for the Republican insurrection and the Tea Party Movement twice over. To begin with, he spent eight years demonstrating that Republicans were at least as willing to increase the size of government, and to do it with borrowed money, as Democrats—indeed, more willing than the most recent Democratic administration. That was a good reason for Republicans who believe in the sorts of things Bush said he believed in to conclude that electing Republicans was no great improvement over electing Democrats, hence that renominating current incumbents would mean the wrong people being elected—whoever won. From there it is a short step to nominating someone else, even at the risk of losing the subsequent election.
Second, and I think equally important, Bush made himself massively unpopular with the electorate, with the result that the Democrats did much better in the 2008 election than they had any business doing, given the distribution of political views in the electorate. Having been handed the White House and large majorities in both houses, along with the excuse of an unusually bad recession and related problems, they proceeded to enact a lengthy wish list of Democratic priorities paid for with borrowed money on an enormous scale. The result, as best I can judge from my very limited political expertise, was that they positioned themselves well to the left of the voters. That set up the current situation.
I will prudently refrain from predictions. It is possible that the Tea Party victories in the primaries will be repeated in the elections. It is also possible, as quite a lot of people have argued, that the Tea Party will turn out to be the best thing that ever happened to the Democratic Party, the one thing capable of saving them in the midterm elections.
We will have to wait and see.