Humphrey entered the race too late to participate in the Democratic primaries. He relied on "favorite son" candidates to win delegates and lobbied for endorsements from powerful bosses to obtain slates of delegates. The other candidates, who strove to win the nomination through popular support, criticized Humphrey's traditional approach. The June 1968 assassination of Robert Kennedy left McCarthy as Humphrey's only major opponent. That changed at the 1968 Democratic National Convention when Senator George McGovern of South Dakota entered the race as the successor of Kennedy. Humphrey won the party's nomination at the Convention on the first ballot, amid riots in Chicago. He selected little-known Senator Edmund Muskie of Maine as his running mate.