Lyndon Johnson strongarmed his way onto the 1960 Democratic ticket. Read the account of Hy Raskin in Dark Side of Camelot by Seymour Hersh, p.124-129:

 Lyndon Johnson Force his way onto the Democratic ticket at the 1960 Democratic Convention - read the account of JFK's close friend Hy Raskin

Read the Dark Side of Camelot by Seymour Hersh, p.124-129:

Close JFK friend Hy Raskin: “Johnson was not being given the slightest bit of consideration by any of the Kennedys… On the stuff I saw it was always Symington who was going to be the vice president. The Kennedy family had approved Symington.” [Hersh, p. 124]

John Kennedy to Clark Clifford on July 13, 1960: “We’ve talked it out – me, dad, Bobby – and we’ve selected Symington as the vice president.” Kennedy asked Clark Clifford to relay that message to Symington “and find out if he’d run.” …”I and Stuart went to bed believing that we had a solid, unequivocal deal with Jack.” [Hersh, p.125]

Hy Raskin: “It was obvious to them that something extraordinary had taken place, as it was to me,” Raskin wrote. “During my entire association with the Kennedys, I could not recall any situation where a decision of major significance had been reversed in such a short period of time…. Bob [Kennedy] had always been involved in every major decision; why not this one, I pondered… I slept little that night.” [Hersh, p. 125]

John Kennedy to Clark Clifford in the morning of July 14, 1960: “I must do something that I have never done before. I made a serious deal and now I have to go back on it. I have no alternative.” Symington was out and Johnson was in. Clifford recalled observing that Kennedy looked as if he’d been up all night.” [Hersh, p. 126]

John Kennedy to Hy Raskin: “You know we had never considered Lyndon, but I was left with no choice. He and Sam Rayburn made it damn clear to me that Lyndon had to be the candidate. Those bastards were trying to frame me. They threatened me with problems and I don’t need more problems. I’m going to have enough problems with Nixon.” [Hersh, p. 126]

Raskin “The substance of this revelation was so astonishing that if it had been revealed to me by another other than Jack or Bob, I would have had trouble accepting it. Why he decided to tell me was still very mysterious, but flattering nonetheless.” [Hersh, p. 126]