I want to believe the policies Trump is pursuing and the people in his cabinet are setting him up for failure and a backlash from voters, but I just don't know anymore. Maybe no matter how bad he does he'll get reelected anyway, like Brownback. Not an apple-to-apple comparison but the last three presidents did get second terms.
Think about it these ways:
1) Trump is going to enter office with worse approval than any POTUS had entering office and with Hillary out of the picture he won't be able to use her as a distraction from his own failings.
2) Trump has made many big promises that will be impossible for him to fulfill. The Wall is physically impossible. The Muslim ban won't get enough votes to pass the Senate. The Rust Belt jobs won't come back. He backed off jailing Hillary because he knows he would fail.
3) Trump saying something bad as a candidate is one thing, but if he says the wrong thing as POTUS, it becomes a different problem because he won't have the same free-speech rights as a private citizen.
4) The GOP is making the mistake of thinking that they can get away with whatever they want so long as they suppress the opposition's vote, forgetting that some of things they have planned will piss off some of their own voters as well. For example, polls show that 75% of the population does NOT want Obamacare repealed.
5) Other than the Alt-Right, the vast majority of the people that actually LIKE Trump are a dying breed of baby boomers. Trump can't expect this fanbase to keep him afloat, especially since it's unlikely he will be able to energize the rural conservatives like he did this year.
6) If Trump is stupid enough to let his advisors convince to go to war with Iran, he will piss off everyone who sees the parallels between that and the Iraq War.
7) Trump better hope that people like John Kasich don't try to Primary him. Kasich showed in the 2016 primaries that Kasich has a much bigger Ohio GOP fanbase than Trump, so there is no risk to Kasich if he tries to Primary Trump.