One of the most tumultuous primary election seasons ever began as the Tet Offensive was launched in January 1968. President Johnson withdrew as a candidate in March, after doing unexpectedly poorly in the New Hampshire primary. In June, Senator Robert F. Kennedy, a Democratic candidate, was assassinated just moments after his victory in the California primary. On the Republican side, Nixon's main opposition was Michigan Governor George Romney, though New York Governor Nelson Rockefeller and California Governor Ronald Reagan each hoped to be nominated in a brokered convention. Nixon secured the nomination on the first ballot. He selected Maryland Governor Spiro Agnew as his running mate, a choice which Nixon believed would unite the party, appealing both to Northern moderates and to Southerners disaffected with the Democrats.