Almost every example of a country walking away from democracy in the modern era falls into one of two categories:
a) a country on a nationalist swing moving (on purpose) away from consultative institutions
(eg: Russia, Germany)
b) a relatively weak country who could be unduly influenced by outside power brokers
(eg: Peru, Argentina, Iran)
This means that any assertion that a president has the ability to steer a country away from democracy is a dubious one. Essentially, no matter how crazy Obama acts, there will be elections. If he acts especially crazy, he will lose much of the backing of congress. I think the lesson of 94 and 06 is that backing bad policy is political suicide. Most legislators went to enough college to understand that.
A more realistic fear for those on the right is that he makes moves that swing our country to the left. That is a real concern. We have seen that Europeans have adopted left leaning ways and then keep reelecting the governments that put them there. I think it is pretty likely that the public will end up liking a European style standard of living and model of healthcare.
