There will come a time (maybe not in our lifetime) where a dictator like figure will not want to relinquish power that easily. I personally don't think Obama has the balls to try it.
It would take a military willing to obey unlawful orders from the CiC for this to happen.
I think the more likely scenario (and I think it is still extremely
unlikely) is martial law following some catastrophe, either natural or manmade, when the Constitutional line of succession is wiped out or disrupted. Some general takes control and decides he likes being in charge. If things are chaotic enough, it might be hard to get rid of him.