Populism. It's practitioners promise the world to the people, but when they get in, they follow every arbitrary rule set up by whatever party that sponsored them, or get nothing done because they aren't working to change the system, or worse they feed every bad trait of populism in that it seeks a villain or villains to blame all of their woes upon.
Trump is like the worst possible combination of all of the above. He has followed every arbitrary Republican rule. if he wasn't before, he became the perfect Republican in pushing through their agenda, he has, despite having complete power over the system, not been overly productive (most of the heavy lifting has been done by congress first when he had the benefit of majority and now when they have the Senate), largely due to courts shutting him down, but that may change long-term because of his (or rather Heritage Foundation, Federalist Society) promotions within the judicial branch, and finally he has so many villains. He is at constant war with someone, or many someones by demonizing them on Twitter. He goes after entire groups of people.
I can't believe I am even saying that we have a President that goes to war with people on Twitter, but that is what we have. He has led us into new and uncharted territory, at least as far as our political system is capable of, and what he is bringing to the table is terrifying. He is scary in that he's the penultimate racist and narcissist and that informs all he does. He is aided and abetted by those who seek his favor, alongside their own wet dreams.