I've been saying this for months. Trump's campaigning style made it easy for his supporters to assign whatever beliefs they wanted to him, to excuse every behavior, and disregard any criticism. His campaign was so grounded in the factless sphere of emotion, that literally anybody who wanted "change" could fudge him into being a good candidate. But as soon as he touches concrete policy, the clock strikes midnight and he turns back into a pumpkin.
The only thing that can end Trump is actually governing, because Trump is no longer being held to some lofty idea of what he might do, but the concrete standard of what he has done. In 4 years, the Democrats are going to be able to lob questions at him from every angle.
Didn't you say you had a secret plan to defeat ISIS? Didn't you say you would keep the US out of overseas conflict? Didn't you promise to make the economy grow at 5%? Didn't you promise to improve healthcare? Didn't you promise not to cut entitlements? Didn't you promise infrastructure spending? On and on and on like that, all the time reminding him that he had 2 or 4 years of uncontested control of the government to get all that done. As it turns out, Trump isn't a political genius. He was just willing to make all the impossible promises politicians usually don't make because they know promises can come back to bite them next election.
Trump knows what people wants, he just doesn't understand how to get there, and rather than asses what is possible in a democracy, he just assumes other people will fill in the details for him. According to Trump, America from 2008-2016 wasn't not great because Obama tried and it was hard, but because he was actively undermining the country and giving away its greatness. Trump truly believes that it's easy, and the only thing standing between America and greatness is having a President that wants greatness. It's delusional
Trump's shtick only works when the question is "what did the last guys screw up, and why should we make a change?" It doesn't work at all when the question is, "what have you done well, and why should you be elected again?" He's already put a trademark on the slogan "Keep America Great" for 2020. The GOP will go from policy blunder to blunder for the next 4 years, mostly because the President couldn't write up a policy solution to anything with two of his brains combined. When it comes to 2020, he's going to have to make the pitch that, despite all the ugliness, America is now bigly bigly greater than it was before. Unless he can solve most of the issues he made promises on in 4 years, who is going to believe that? And if the economy tanks in the meantime, will anybody even think twice about it?