Matt Stoller has a very well thought argument given the progressive case against Obama:

So why oppose Obama? Simply, it is the shape of the society Obama is crafting that I oppose, and I intend to hold him responsible, such as I can, for his actions in creating it. Many Democrats are disappointed in Obama. Some feel he’s a good president with a bad Congress. Some feel he’s a good man, trying to do the right thing, but not bold enough. Others think it’s just the system, that anyone would do what he did. I will get to each of these sentiments, and pragmatic questions around the election, but I think it’s important to be grounded in policy outcomes. Not, what did Obama try to do, in his heart of hearts? But what kind of America has he actually delivered?

Obama had a number of things that he could do without the Consent of congress, holding torturers accountable, prosecuting corrupt bankers, etc.  In every case, his decisions have served to further empower, and to further immunize, powerful wrongdoers in our society.

I recommend that you read the whole article.

A counter argument by Joss Weadon(see vid) involves Zombies