Comparing Obama's decline in approval ratings to other 20th century presidents results in some surprises.
The good folks at TPM have done the obvious and yet incredibly helpful and looked at the month-to-month approval ratings of all the Presidents since Eisenhower. You can see it much larger and clickable here. But here are some takeaways:
Obama is not doing very well. Only three presidents had a worse approval rating than him at this point, and one was Gerald Ford. However, two of those presidents below him went on to win second terms. The biggest point, though, is that all presidents are in basically a constant approval decline, with only small spikes (often beyond their control, see below) to break the continual downward trend. The highs of the election, and excitement about election promises, seem to only lead to disappointment and disillusionment. Maybe we would be better served by campaigns and candidates that were realistic about what they planned to do? Then again, that doesn't seem like something voters would get excited about.
Here are some more specific charts:
Obama vs. George W. Bush. As you can see, Bush got a huge lift from 9/11, though quickly began descending from those heights.
Obama vs. Clinton.
Obama vs. Reagan.
TPM has explanations of the various ups and downs of each presidency. Check out the full data there.