Most economists believe the economy will grow at about a 3% annual rate between now and November 2016. If that happens, my equation predicts the Democrats will win about 46% of the vote in a two-party contest. In order for them to win more than 50%, the economy would need to grow at about 4%, and even in that case their predicted vote share would climb only slightly above the halfway mark.
My equation's average prediction error over the 25 elections since 1916 is between 2.5 and 3.5 percentage points. Assuming the economy does indeed grow at 3%, the probability that the Democrats will win is low, between about 5% and 13%. Republicans have cause for confidence.
My analysis is, of course, based on the assumption that the future will be like the past. What if voters start caring more about income inequality than economic growth, and perceive the Republicans to be poor on that issue? Or what if the GOP nominates someone further from the mainstream than ever before?
Shifts in priorities can never be ruled out, but the best I can say is that the conditions that sway voters appear to have been fairly stable for 100 years — and my equation has a fairly good track record.