I mean this to be for someone who has secured the nomination of their party, but I'm also interested in hearing about people who were leading the polls towards their eventual nomination, but died before they could be declared the winner.
I think there are two examples. Horace Greeley was the Democratic nominee in 1872 and died after the popular election (but before electors voted). He lost the election anyway, so it didn't really affect anything. He still received the electoral votes.
James Sherman was President Taft's vice president, and had been renominated in 1912. He died a week before the election. Taft was left without a running mate. The RNC quickly nominated Nicholas Butler to the ticket, but they lost the election anyway.
As for non-nominees, the obvious figure is Bobby Kennedy. He was extremely popular in the election of 1968, and was assassinated. This was during the primary season, so RFK actually had some delegates. Ultimately, the liberal supporters of RFK drifted to Eugene McCarthy who then won the nomination.