Is it normal that democrats havent succeeded two democratic terms since 1836 ?
History seems to think so!
1836, the Democratic Vice President Martin Van Buren succeeded Andrew Jackson by defeating four Whig candidates
The only other time since 1836 is when President Franklin D. Roosevelt succeeded himself in 1940 by running for an unprecedented third term. (This one doesnt really count now since he was only succeeded by himself, not by a different Democrat, and now we have a strict two-term limit for Presidents)
Since then, they never succeed past two terms.
So i'd say the probability of a Democrat winning in 2016 is next to nil.
Republicans, on the other hand, tend to hold power longer and have more Republican successors. So it is more likely that when Republicans win in 2016, they won't lose the Presidency again until 2024 at minimum, but maybe 2028 or later !