The burden of proof is not on the person making the ordinary, obviously indicated, long accepted, and overwhelmingly evidence based observation. All the Birthers, for example, have presented overwhelming evidence of racism by adopting that political stance in public. Trump is their leader. Likewise with the Pocahontas jokes about Elizabeth Warren, the Mexican rapist and terrorist lines, the hostility to BLM and resistance to the ordinary accounts of reliable black witnesses, and so forth. Any Trump backer is signed on with that crowd and with Trump himself - burden of proof thereby acquired. It's the extraordinary claim that requires the extraordinary evidence - the "proof".