If the democrats run another extremest, Trump will probably win. In the unlikely event that they choose to run a centrist candidate, they may unseat him.
I wouldn't put anything stupid past the masses... but what won Trump the election before (minus Russia, of course) was they sowed just enough doubt about Hilary that people chose to not vote instead of vote for their party. If he can instill enough fear, then yes. But honestly, I am more afraid of what he will do if he doesn't get reelected.
i don't see voters swinging the populist pendulum to the far left. American's are still in love with beating up immigrants and minorities, protecting big corporations and growing the 1%. i think President Trump will return for a second term.
full disclosure: as a republican I have no idea which piece of shit politician i'll cast my vote for next year - maybe i'll vote democrat just to reduce the republicans to a minority government...idk