Of course. Even vs. a popular Trump, they have a CHANCE. Trump was barely elected, and while he'll enjoy incumbency, he's still polarizing enough to possibly face an upset. If he's underwater with popularity, I think he'll be a slight underdog. If he's at Bush levels (not entirely improbable), he could even face this kind of a defeat as a worst case scenario vs. a strong Democrat with a populist message:
This is definitely possible. What we need is a candidate that has swagger and isn't just a Civics nerd. That is why Kerry and Romney lost against controversial to slightly unpopular presidents. When was the last time someone who wasn't new or different won the presidency? HW? But he was just a protege of someone who was new or different. Nixon? Was FDR considered?