At the start of 1968, most Democrats expected that President Lyndon B. Johnson would be re-nominated. Those expectations were shattered by Senator Eugene McCarthy, who centered his campaign on opposition to Johnson's Vietnam policies. McCarthy narrowly lost to Johnson in the first Democratic Party primary on March 12 in New Hampshire, and the closeness of the results startled the party establishment and spurred Senator Robert F. Kennedy of New York to enter the race. Two weeks later, Johnson told a stunned nation that he would not seek a second term. In the weeks that followed, much of the momentum that had been moving the McCarthy campaign forward shifted toward Kennedy. Vice President Hubert Humphrey declared his own candidacy, drawing support from many of Johnson's supporters. Kennedy was assassinated by Sirhan Sirhan in June 1968, leaving Humphrey and McCarthy as the two remaining major candidates in the race. Humphrey won the presidential nomination at the August Democratic National Convention in Chicago, and Senator Edmund Muskie of Maine was selected as his running mate. Outside the convention hall, thousands of young antiwar activists who had gathered to protest the Vietnam War clashed violently with police. The mayhem, which had been broadcast to the world in television, crippled the Humphrey campaign. Post-convention Labor Day surveys had Humphrey trailing Nixon by more than 20 percentage points.