Also worth noting: Republicans have only once won the popular vote since the turn of this century, in 2004 for George W. Bush's reelection, when he had both the incumbent advantage and was still riding the post-9/11 patriotism wave
or put another way, the democratic candidate have won the popular vote on 5/6 presidential elections this century
It's funny because here being incumbent usually is a disadvantage because you get blamed by all the crap that's happening, even the little that isn't their fault.
The median voter is woefully uninformed and largely votes on vibes and name recognition. Many fail to understand exactly what kind of power the president actually holds.
Personally I blame this in part on the death of journalism. Local newspapers keep going out of business which removes any accountability for local authorities, and the only way you know of anything happening is based on Facebook gossip dripping in all of the biases the individuals who are there when something happens. And the local news that still exists keeps getting bought up by larger entities that may or may not be politically motivated to try to sway opinions and set the conversation across the country. Or worse in some cases independent news outlets are simply threatened into not investigating or reporting on certain topics