Picking Kamala Harris as Vice President. Resulted in about 10 million voters voting for Trump who otherwise would have stayed home. Cost him NC, FL, TX and ME-02; and cost Senate Dems NC, ME, and IA. Cost US House Dems 32 seats.
This seems like an exaggeration as there's almost no way Biden would've won Texas or ME-2 no matter which VP he chose, with the possible exception if he chose Jared Golden for the later but that would've cost him in other states. I think Val Demings would've been the best VP Biden could find, especially contrasting with Kamala'a rich lawyer vibe. She would've had all of Kamala's strengths and her relatively moderate profile would've likely flipped North Carolina and at the least put him within a % of flipping back Florida.
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