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She set a tone at the very start when she walked right into his space to shake his hand and made him almost pull back into himself in response. She was in charge and never stopped being in charge.
Harris also managed what neither Joe Biden nor Hillary Clinton nor any of the 2016 Republicans managed to do which is successfully bait Donald Trump and get under his skin. Within a few minutes Trump was visibly angry and not in a way that empowered him but in a way that made him lose focus, go down rabbit holes and generally go off onto damaging tangents. Spittle anger, not righteous anger, shall we say.
Because his base apparently absolutely eats it up. They don’t like nuance. They like crisis and outrage and hate and things they don’t have to think too much about or have gray areas. They are purely good, the enemy is pure evil, etc.
The country is collapsing, she is the worst ever, etc. It’s easy.
Not all of his followers are like that, but holy shit is it way more than I ever would have guessed. And it works so goddamn well.