How did it come about that popular opinion seems to hold that hatred exists predominantly on the ideological right, even though historically, and even contemporaneously, the left is as equally riven with hateful rhetoric and deeds as is the right, and quite arguably much more.
Historians can probably point to several events in which the "political right = haters" had its genesis, but one contender for the most salient example was the propaganda following the assassination of President John F. Kennedy in 1963.
Kennedy was an icon on the left despite pushing economic and foreign policies that were anything but compatible with leftist ideology.
According to Jonah Goldberg writing in his excellent book Liberal Fascism, Kennedy campaigned on a the need for stronger military defense. He tried to depose Fidel Castro, the communist dictator of Cuba, and presided over the Bay of Pigs fiasco. He brought the world to the threshold of nuclear war in the Cuban missile crisis, and got us deep into a pointless war in Vietnam before he was assassinated.
He also cut taxes, and although he championed civil rights legislation for blacks, he was simply building on what Republicans had started in the 1950s under Dwight Eisenhower.
Nevertheless, he became posthumously a mythical character among liberals. When he was shot, the media, particularly people like CBS reporter Dan Rather, were quick to blame the political right for the crime and brand Dallas, Texas, the city and state in which the assassination occurred, the loci of hate in America.
When it turned out that the shooter, Lee Harvey Oswald, was in fact a Marxist, a man of the left, liberals were stunned. They proceeded, however, to nourish the Kennedy mystique, ascribing heroic traits to the man that he never possessed in real life and claiming that anyone (i.e. Republicans) who opposed him politically must've been motivated by hatred.
The irony is that Kennedy was much closer ideologically to contemporary Republicans than he would've been to contemporary Democrats, or even to Democrats of the 1960s.
The point, though, is that Kennedy hagiography became a big business. He was turned into a martyr by his party and a media which, always eager to be seduced by superficiality, adored him for his charmingly suave persona.
In order to achieve his canonization, however, his opponents had to be vilified and it was convenient to accomplish this by convincing people that his opponents were "haters."
The ruse worked, and continued to work for several decades thereafter, largely because people got their political opinions from relatively few sources, almost all of which were committed to propagating the myth of Kennedy sainthood and right-wing hatred.
Even today, the left continues to push the meme, labelling much that Republicans, especially the president, say as hateful, racist, bigoted, etc., and people who lack the time or the inclination to ask what, exactly, is hateful about whatever it is that has been called such by the media or one's Facebook friends, simply accept that it must be.
It would be good for all of us to think more critically when these terms are tossed about, to ask, when someone calls someone else a racist, what their definition of a racist is and how and why it applies to the individual in question.
Many times we'll find that those using these labels have no good answer to these questions. They simply use them as rhetorical vehicles for expressing their dislike for the other and because it saves them the hard work of having to develop arguments and to actually think.