The strongest argument that the Republican Party can make in favor of candidate Mitt Romney is that he isn't Barack Obama. He most certainly was not the candidate favored by the Tea Party or many other more conservative Republicans, in spite of efforts to paint him as such. The nomination of Mitt Romney was a fairly decisive defeat of the Tea Party's influence within the Republican Party and a victory for the central leadership over the base.
The strongest argument that the Democratic Party can make in favor of candidate Barack Obama is that he isn't Mitt Romney. He fails to inspire the base, even with his alleged achievements. The strongest argument in favor of Barack Obama is potential judicial appointments, as Mitt Romney would appoint people like John Roberts who voted to uphold Obamacare. "Hope and change" makes a good mantra when a candidate is fresh, but not after failing to deliver either for four years.
Small wonder that partisans react so angrily to having it pointed out how little difference there is in this campaign. The standard comparison of "not a dimes worth of difference" is even stronger given just how similar the two candidates really are. Mitt Romney wrote the rough draft of Obamacare, and Barack Obama has shown greater militarism than even George W. Bush.
The cries of "this is the most important election ever" are even louder than before, and yet those cries fail to inspire the base of either party. These cries are meant to convince the reluctant base to come out and vote in spite of, not because of, the candidates that the party is running. They are also a vain attempt to convince third party voters to cross party lines, which sometimes does happen if a candidate is appealing enough. In the 2012 electoral race, it would be hard to describe either candidate as "appealing enough."
It will be interesting to see how a campaign between two unappealing candidates develops. Negative campaigning, trying to appeal to fear in order to bring out the base, will be the only option for both candidates, but ultimately that will reinforce the basic problem that neither candidate is good. It will also be interesting to see how the two parties arrange to negate the opportunity being given to third parties this time around.