By 1967 social turmoil and the mishandling of the Vietnam War were making a second full term for LBJ more and more inoperable for Democrats. Baby boomer students and hard New Left radical elements created a fissure so deep with Souther Democrats, working-class democrats, and the party establishment that it made the internecine division in the Republican party seem quaint. Violence had exploded across the nation. The anti-war protests along with New Left Marxian opportunists strategically embedded in the movement were turning off a growing number of Americans. The country seemed to be spinning out of control and the Democrat party was right at the center of the disarray.
The last straw for LBJ was the announcement that Robert Kennedy was running. Within two weeks, a politically savvy President Johnson understood the writing on the wall. After already barely beating Sen. Eugene McCarthy in the New Hampshire primaries, he famously announced he would neither seek nor accept the nomination of the party on March 31st. The field was now open. Sort of.
Sixty-six days later, Robert F. Kennedy was murdered at the Ambassador Hotel in Los Angeles immediately after celebrating his win in the California presidential primary against Vice President Hubert H. Humphrey. The Johnson Administration had thus inherited two nominations from Kennedy assassinations in 5 years' time, as Humphrey eventually became Nixon's general election opponent. But the Democrats were impossibly fractured by this point, and Nixon inevitably won in November.