This was not a referendum on Obama's first year. It was rather a slap at the Congress. They are the ones who have to be concerned, especially, if they are up for re-election in November. The voters in 2008, voted for change. What they got was more of the same. In his acceptance speech, Brown stated quite correctly that "the independent majority has delivered a great victory". The Republicans will claim this as a victory, but they should be wary of basking too long and should instead learn from this, but, being conservative and Republican and with little to offer, they will trumpet their victory and call it an indictment of Obama's agenda.
More and more Americans are shelving their party credentials and becoming Independents. This means that all bets are off as far as party loyalty. Sure there are those who will contionue to vote Democrat or Republican, but the decissive vote is out there floating as independent and they will make the difference.