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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.
I wasn’t shocked at all (except the general shock that a person like this has ever come close to the presidency). Particularly in recent years, I’ve seen him take bait so many times which wasn’t even intended as bait, but it triggered him. And because it slightly grazed his fragile ego, priority number one for him is to go on an ego-defending tangent that only confuses and undermines any points he could have made about the actual topic.
Harris knew that as long as she peppered her responses with a trigger here and a trigger there, he would be unable to help himself every time. And that’s what we saw.
The entire reason he ran in the first place was because Obama made a joke about him in a speech, and it embarrassed him.