For the final 41 of those 50 years, following the assassination of Robert Kennedy, his younger brother Ted was not only the leader of a political family, but a synecdoche for American liberalism. Conservative candidates and organizations raised millions of dollars using Kennedy's image and words in direct mailings. Kennedy was the nation's leading liberal for so long that it seems obvious that the conservative opposition to liberalism was identical to its opposition to Kennedyism.
The story of the entire half-century is a little different, and more interesting. During John Kennedy's 34 months in the White House there were a number of signals that both he and his brother/consigliere Bobby couldn't stand liberals for the same reason that conservatives couldn't, and can't. Specifically, JFK seemed to disdain the sob-sister liberalism of Eleanor Roosevelt and Hubert Humphrey, which collapsed the distinction between politics and social work. The Kennedy administration, instead, was supposed to usher in the age of "liberalism without tears." Kennedy also seemed to disdain the high-minded dithering of Adlai Stevenson and the "amateur Democrats" who idolized him. Kennedy liberals prided themselves on being tough, decisive and vigorous. They were professionals.
Kennedy's admirers praised the tone of cool irony he brought to national politics. Conservatives weren't among those admirers, but felt that JFK's irony operated at only one remove from his cynicism, which they found reassuring. As president, Kennedy delivered Ted Sorenson's resonant lines about domestic policy impressively, but used or risked very little political capital to advance the liberal domestic agenda. His involvement in the civil rights issue, for example, was conspicuously cautious, even reluctant. It was clear, during his presidency, that Kennedy was no crusader, and far from clear what he really cared about and wanted to accomplish.
After Dallas, however, all of that ironic detachment was transformed into moral urgency. The rhetoric about which JFK had seemed so equivocal and done so little was transformed into sacred scripture. As James Piereson has argued, Kennedy's family and retainers began an aggressive campaign to turn his murder into a politically resonant tragedy, one that would see him remembered "as a martyr for civil rights and equal justice for all." Sen. Mike Mansfield, the Democrats' majority leader, said in his eulogy, "He gave us his love that we, too, in turn, might give. He gave that we might give of ourselves, that we might give to one another until there would be no room for the bigotry, the hatred, prejudice and the arrogance which converged in that moment of horror to strike him down." For the record, Mansfield was speaking of Kennedy's assassination rather than Christ's crucifixion.
"Once having accepted the claim that Kennedy was a victim of the national culture," writes Piereson, "many found it all too easy to extend the metaphor into other areas of life, from race and poverty to the treatment of women to the struggle against communism." Dallas saw the demise of liberalism without tears, which was replaced by a liberalism regularly operating at the brink of hysteria. Both the surviving Kennedy brothers got swept up in it. It wasn't sufficient for Bobby Kennedy to say that the war in Vietnam he had helped his brother launch was a mistake, or that the national interest would be better served by choosing a more promising and important battlefront for repelling Communist aggression. Rather, he told an audience in 1967 that what America was doing in Vietnam was not that much different from what Hitler did to the Jews. Nineteen years later, Ted Kennedy was equally fair-minded in declaring his opposition to Robert Bork's nomination to the Supreme Court: "Robert Bork's America is a land in which women would be forced into back-alley abortions, blacks would sit at segregated lunch counters, rogue police could break down citizens' doors in midnight raids, children could not be taught about evolution."
John Kennedy's death started an era where that sort of rhetoric was common and even obligatory. It would be a step forward if Ted Kennedy's death marked the beginning of an era where national issues, even the most important ones, are debated in the belief that decent, reasonable and intelligent people can disagree.