If there is one consistent thread through Mr. Trump’s political career, it is his overt connection to white resentment and white nationalism. Mr. Trump’s fixation on Barack Obama’s birth certificate gave him the white nationalist street cred that no other Republican candidate could match, and that credibility has sustained him in office — no amount of scandal or evidence of incompetence will undermine his white supremacist followers’ belief that he, and he alone, could Make America White Again.