reference
The US presidential primary process, explained
"After the 1968 fiasco, the Democratic National Committee created a commission charged with proposing reforms to the DNC nominating process."

Calls to Reform Democratic Nomination Process Have Roots in 1968 Convention
"'A lot of things we take for granted today are the result of the changes that were made that year," USC Prof. Geoffrey Cowan tells TIME. 'The system that, let's say, Donald Trump and maybe some others call 'rigged'—it was really rigged before then.'"