A safe bet
Here’s something to chew on. Trump did not win the election as much as the Democrats were almost certain to lose the presidency, regardless of whom they put forth as their candidate.
In the past 60 plus years there has only been one instance in America where the incumbent party earned a “third term” and that was when George H.W. Bush succeeded Reagan. People who specialize in divining the roots of presidential elections say the plain fact of the matter is that after eight years people want change, out with the old and in with the new. That in large measure is what happened.
A year ago you could have given two to one odds that the Republicans would win the 2016 presidential election with a high assurance that you made a safe bet. Sorry to say, almost everything else is statistically irrelevant as we witnessed a divisive individual such as Trump hijack the Republican Party nomination and then win the general election.
It helped, of course, that the Democrats chose a deeply flawed candidate.