It all started with the 1968 Democratic Convention.
The lead up to the convention had been tumultuous. The Vietnam War was in its 14th year, both Martin Luther King, Jr. and presidential candidate Robert Kennedy were assassinated that spring and President Lyndon B. Johnson had withdrawn from the race in March, deciding against seeking another term. That April, Hubert Humphrey—Johnson’s vice president—jumped into the race. Humphrey’s public support of Johnson, specifically regarding the Vietnam War, upset many anti-war protestors.
While Democratic political leaders filed into the convention hall, protestors brutally clashed with police right outside its doors, with television broadcasting the political divide to the nation. Hubert Humphrey would go on to win the Democratic nomination (over George McGovern and Eugene McCarthy) despite not winning a single primary, highlighting for many the disparity between public opinion and the political process.