McCarthy's strong finish in the New Hampshire primary showed the vulnerability of the incumbent president. It also convinced another war opponent to enter the race.
Democratic Sen. Robert Kennedy of New York, the 42-year-old brother of the late president, made his announcement just four days after New Hampshire.
"I'm announcing today my candidacy for the presidency of the United States," Kennedy said on March 16, 1968. "I do not run for the presidency merely to oppose any man, but to propose new policies."
KENNEDY'S ENTRY CREATES HAVOC FOR MCCARTHY
Five months earlier, McCarthy had tried unsuccessfully to convince Kennedy to challenge Johnson. He deeply resented Kennedy's late change of heart, as he revealed in an interview the following day, March 17, 1968.
"Senator McCarthy, do you like Robert Kennedy?" the host asked.
"Well, I -- as someone said, I like everybody I know. You have to like people you know," McCarthy responded. "The question of other judgments you make about people as politicians and their procedures as politicians, but as a person..."
"Do you like him as a politician, sir?" the host pressed.
"Well, I've, uh, I would advise him to have acted differently in the last three or four days as a politician. So that in kind of an objective, detatched judgment you pass on politicians, you distinguish from persons," he said. "I would say that I don't altogether approve of what he's done in the last three or four days, not just as it bears upon me but in terms of what I think is a common cause that we do have."
Kennedy's entry fractured the McCarthy campaign. Many of McCarthy's young workers admired and supported the New York senator.
Ann Hart was organizing the McCarthy effort in California at the time. She talked about the impact of the Kennedy decision during a 1969 oral history interview.
"Senator McCarthy was not exactly, you know, warm and amiable about Robert. And it all siphoned down to the campaign," Hart recalled. "It got to the point where if you were suspected for even having any good feelings about Robert Kennedy at all, you were expelled from the group as an unclean person."
Many peace advocates and civil rights leaders welcomed Kennedy to the race. The Rev. Martin Luther King, Jr., didn't endorse either candidate, but he was pleased there were two men challenging Johnson and the Vietnam War.
"I think both Mr. Kennedy and Mr. McCarthy represent the kind of competence, dedication and relevant thinking on the basic issues that confront us today, that they are the kind of men that present the alternative that I think we need," King said.
LBJ DROPS A BOMBSHELL
The Kennedy announcement disrupted the 1968 campaign, but it paled in comparison to the one President Johnson made two weeks later.
Johnson went on national television to announce a pause in the bombing and to call for peace negotiations with the North Vietnamese. Then came his surprise ending.
"I shall not seek, and I will not accept, the nomination of my party for another term as your president," he said.
Johnson's announcement, just two days before his primary defeat in Wisconsin, turned the presidential race upside down. His withdrawal was one of many events that shocked the nation in 1968.