 Complete the following phrase:  The Robert F. Kennedy presidential campaign began on March 16, 1968, when Robert Francis Kennedy  RFK, Bobby, a United States Senator from New York who had won a Senate seat in 1964, entered an unlikely primary election as a challenger to incumbent Democratic United States President Lyndon B. Johnson LBJ. After Johnson announced on March 31 that he would not seek re-election, Kennedy still faced two rival candidates for the Democratic Partys presidential nomination: the leading challenger United States Senator Eugene McCarthy and Vice President Hubert Humphrey. Humphrey had entered the race after Johnsons withdrawal, but Kennedy and McCarthy remained the main challengers to the policies of the Johnson administration. During the spring of 1968, Kennedy campaigned in presidential primary elections throughout the United States. Kennedys campaign was especially active in Indiana, Nebraska, Oregon, South Dakota, California, and Washington, D.C. He had been making progress in building Democratic support for his nomination when he was 