At 538.com, Tom Schaller points out a reasonably obvious but oft-forgotten reason for the steady erosion of President Obama’s approval ratings over the last few months: he’s trying to do a lot, and trying to do a lot depletes political capital more rapidly than trying to do very little:

Big change is costly, and not just in actual dollars from the Treasury, but in terms of how much of his capital reserves a president is willing to spend to get what he wants. Obama is not plugging for school uniforms, folks. He’s re-regulating Wall Street, trying to stimulate the economy by pumping nearly $1 trillion into it, and attempting to tackle the policy problem too many of his predecessors never could: reforming our messy, complicated health care system. Accordingly, he’s paying the price for even trying.

To put it another way, what would you prefer if you are a Democrat: that the President keep his approval ratings above 60 without working to implement the agenda he campaigned on, or lose some points and mybe get something done that won’t have to be done later or left undone entirely?
No gain without pain, folks.