When the Republican establishment voted for a Democrat
Barry Goldwater's nomination for Republican president in 1964 was too much for some in the party establishment, who balked at his "extremism". Like Trump and Cruz, Goldwater dragged his party to the right. Nelson Rockefeller, who was the front runner until his high profile marriage to a younger woman cost the New York governor his early lead, was portrayed by an actor in this devastating campaign ad, in which he announced his support for Lyndon Johnson.
Random Nerds blog goes into detail about all of this and can be accessed here.