Someone on GAW introduced me to the "Ford Play" yesterday. Nixon's VP was originally Spiro Agnew. Agnew resigned in 1973 over an income tax scandal and was replaced by Gerald Ford. Less than a year later, Nixon resigned and Ford became POTUS without ever receiving a vote. This is something to keep in mind.

edit: There is potentially a Constitutional loophole that could even allow for Obama himself to become POTUS for a third term. The 22nd amendment prohibits anyone from being "ELECTED" to POTUS more than twice, but as you can see from the Ford Play above, Ford was never elected to POTUS at all. Something else to keep in mind.

edit 2: since I got stickied (Thanks), I want to add that I'm not making a prediction here at all. Even if research on this idea checks out, I would still say that it's only one of many possible DS plans. Not all plans get used. Not all plans work out. Just something to keep in mind.