Suppose that the major-party nominees for president had been nondescript, people like Mike Pence or Tim Kaine. In that case, it seems that the outcome would depend on "how things are going"--if they're going well, keep a Democrat in office; if not, turn to the Republican. There have been a number of efforts to figure out exactly which "things" matter and how much difference they make. One of the best-known ones was proposed by Ray Fair in 1978 and has been modified and updated since then. In Fair's formula, support for the incumbent party depends (positively) on the rate of growth of per-capita GDP in the first three quarters of the election year and the number of quarters of rapid growth over the president's term and (negatively) on the rate of change in prices over president's term. Of course, we don't know what those will be yet (currently the GDP data only goes through the first quarter of 2016), but if you put in his predictions of what those will be, the equation predicts the Republican would get 56% of the two-party vote. Even if you assume very strong (4%) growth in the second and third quarters, that only brings the Republican share down to 52.7%.
So according to his formula, the objective conditions put the Democrats at a big disadvantage. Why? Part of the reason is that there have been few quarters of rapid GDP growth over the last term--as many people have pointed out, the economic recovery has been slow. But that was also true in 2012, when the formula predicted that Barack Obama would win the popular vote. The main reason for the difference is that the formula also includes variables for incumbency and for the duration of time the party has been in power. Incumbency is estimated to increase vote share by 3.0%, and being in power eight years rather than four is estimated to reduce it by 3.8%. Putting those together, the Democratic candidate in 2016 starts off 6.8% worse than Barack Obama did in 2012, and 7.6% worse than Obama did in 2008 (when duration was counting against the Republicans), before accounting for economic conditions. These are big differences, both in absolute and relative terms (bigger than the difference that would result from a -5% rate of growth compared to a +5%).
When I checked earlier today, the Republican share of the two-party vote in the latest polls was about 47.5. If that holds up, it will be the largest prediction error in the period covered by the estimates--the current record is 1992, when George HW Bush was predicted to get 53.6% of the two-party vote, but actually received only 48.3%.