 In early June , Nixon continued to be regarded as the favorite to win the nomination , but observers noted that he had not yet locked up the nomination . He still faced challenges from Nelson Rockefeller and Ronald Reagan , and was not on the ballot in California , where Reagan won a large slate of delegates . Behind the scenes , Nixon staff lobbied for delegates from " favorite son " candidates , resulting , in the backing of Senator Howard Baker of Tennessee , and his 28 pledged delegates , as well as those 58 delegates supporting Senator Charles Percy of Illinois . After the assassination of Robert Kennedy , like the other candidates , Nixon took a break from campaigning . Reports suggested that the assassination all but assured his nomination . Upon returning to the trail , Nixon found that Rockefeller had begun attacking him . Rockefeller described Nixon as a man " of the old politics " who has " great natural capacity not to do the right thing , especially under pressure . " Nixon refused to respond to the jabs , stating that he would not participate in attacks . As he edged closer to the nomination , discussions about his running mate arose . Republicans in the Midwest pushed for Mayor John Lindsay of New York City . The endorsement of Nixon by Senator Mark Hatfield of Oregon raised speculation that he might be chosen . Congressman George Bush of Texas and Senator Percy were also mentioned as possible selections . At the end of the month , Nixon had two thirds of the required 667 delegates necessary to win the nomination . 
