Sen. Robert F. Kennedy was assassinated in 1968.

Jordanian refugee Sirhan Sirhan, 24, shot Kennedy after a speech at the Ambassador Hotel in Los Angeles.

After death, Kennedy became an icon for the Democratic Party. He was seen as a potentially unifying force for the country, since a lot of his campaign had focused on civil rights.

The impact of his death is harder to gauge than others on this list, but he had just won the California primary, and was gunning for the Democratic presidential nomination.

Although Joshua Zeitz wrote for Politico, "The hard reality is this: Bobby Kennedy would most likely not have won the presidency, because he was already on track to lose his party's nomination."