Bryan Cranston hits the nail on the head regarding the "Make America Great Again" movement.
Just because white Americans think that the past was "great" doesn't mean it was to ALL Americans. Civil rights in this nation have been progressively moving forward, and although racial equality is nowhere near where it should be in America, we are certainly a lot better off than we were in generations past.
When many African Americans hear the slogan "Make America Great Again" it certainly doesn't exude hope of a brighter future. In fact it likely puts dread into their minds that we could revert back to the 1950's or even worse the 1860's.
So when people point to MAGA as being a racist dogwhistle, they certainly have an argument as to why it can at least appear that way to certain groups of Americans.