Nixon was an affront to the hard left because he'd nailed Hiss.
For this he had to be destroyed.
Nixon was an affront to the "mainstream" left because he'd turned around the situation in Vietnam in the space of just a few years (largely by doing the bloody obvious, e.g. destroying North Vietnamese bases in Cambodia and Laos and bombing the crap out of Hanoi) after the 'Rats and RINOS had pissed around with the war for nearly a decade.
For this not only did Nixon have to be destroyed, but so did the honorable peace he had achieved for South Vietnam. You can see Kerry embracing that goal, quite openly, in this very video tape. (See also my msg #277.)
The "mainstream" left was, in this, even more treacherous than the hard left. At least the hard left always wanted the communists to win. At least they strove consistently to undermine American support for any non-communist government in South Vietnam. The "mainstream" left, OTOH, overwhelming supported the defense of South Vietnam (indeed they drove the escalation) until a Republican was elected.
The 'Rat congress defaulted on our solemn promises to Vietnam, and threw an entire country with it's millions to the wolves, largely out of their hatred for one man. And that hatred, although it was fed by many streams, stemmed in large measure from the fact that he successfully conducted and concluded a war that they had botched horribly.
See some parallels to current events there? You damn well better, because the left won't hesitate to do the same thing again.