Though President Lyndon B. Johnson had served during two presidential terms, the 22nd Amendment did not disqualify Johnson from running for another term, because he had only served 14 months following John F. Kennedy's assassination before being sworn in for his 'full' term in January 1965. As a result, it was widely assumed when 1968 began that President Johnson would be a Democratic nominee, and that he would have little trouble in winning the Democratic nomination.
Despite the growing opposition to Johnson's policies in Vietnam in both Congress and in the public, no prominent Democratic politician was prepared to run against a sitting President of his own party. Anti-war activists of the new "Dump Johnson movement" initially approached Senator Robert F. Kennedy of New York, an outspoken critic of Johnson's policies with a large base of support, for a candidacy, but he declined to run. They then appealed to Senator Eugene McCarthy of Minnesota, who was willing to openly challenge Johnson. Running as an anti-war candidate in the New Hampshire primary, McCarthy hoped to pressure the Democrats into publicly opposing the Vietnam War. Trailing badly in national polls and with little chance to influence delegate selection absent primary wins, McCarthy decided to pour most of his resources into New Hampshire, the first state to hold a primary election. He was boosted by thousands of young college students who volunteered throughout the state, who shaved their beards and cut their hair to be "Clean for Gene."
On March 12, McCarthy won 42% of the primary vote to Johnson's 49%, an extremely strong showing for such a challenger, and one which gave McCarthy's campaign legitimacy and momentum. In a surprise move on March 16, Robert F. Kennedy renounced his earlier support for Johnson and proclaimed his candidacy. McCarthy and his supporters viewed this as opportunism, creating a lasting enmity between the campaigns.
Johnson was now faced with two strong primary challenges. In declining health and facing bleak political forecasts in the upcoming primaries, Johnson concluded that he could not win the nomination without a major political and personal struggle. On March 31, 1968, at the end of a televised address regarding the War, the President shocked the nation by announcing that he would not seek re-election. By withdrawing from the race, he could avoid the stigma of defeat and could keep control of the party machinery to support Hubert Humphrey, his loyal Vice President. As the year developed, it also became clear that Johnson believed he could secure his place in the history books by ending the war before the election in November, thus giving Humphrey the boost he would need to win.
With Johnson's withdrawal, the New Deal Coalition effectively dissolved into support for different candidates:
- Hubert Humphrey, Johnson's Vice-President, gained the support of labor unions and big-city party bosses (such as Chicago Mayor Richard J. Daley), who had been the Democratic Party's primary power base since the days of President Franklin D. Roosevelt. It was also believed that President Johnson himself was covertly supporting Humphrey, despite public claims of neutrality.
- McCarthy rallied students and intellectuals who had been the early activists against the war in Vietnam;
- Kennedy gained the support of the poor, Catholics, African-Americans, and other racial and ethnic minorities;
- Conservative white Southern Democrats, or "Dixiecrats," their influence declining swiftly in the national party, tended to support either Vice-President Humphrey or George C. Wallace and the Alabama governor's third-party campaign in the general election.