I take my lessons from history and you are right but there is one opponent that cannot be defeated and its a perfectly naturally occurring one which is the prospect of recession and an economy that forces votes to change that otherwise might have stayed loyal to the President.  Bush Snr. vs Clinton is the precedent for this.
   People will stick with what works, simple as that and you cant blame them but if people are literally being put out of work it causes people to stop and think and have plenty of time to do so even.  They are far more likely to be swayed to a new candidate when this is the case rather then an 'alls well' situation.   Obviously theres always some bad news, I watch it on the TV every night and its sad some of it but what effects me most is my wallet and if I can spend and pay the bills or not, thats just the personal motivation every voter shares.
   Any great consensus forming is where trend change is possible despite the bias to a reelection, its the same in every consideration for every market every nation pretty much.   If you want to take the outside odds on this bet, consider the prospect of two quarters of GDP falling around the election timing and if possible thats a viable bet I would argue.