Well no, ever hear of Eugene McCarthy? However "Clean Gene" was not, AFAIK, collaborating with the enemies of the US when he ran against LBJ in the primaries. His strong showing in New Hampshire brought Fat Red Ted's brother Robert into the race. That set Bobbie up for Sirhan Sirhan (A Palestinian, BTW) leading to the nomination of LBJ's VP, Humphrey, who then lost to "Tricky Dick" (Ike's VP). RMN had previously been beaten by JFK (Kennedy, not Kerri)
Thanks, I was well aware of the '68 primaries, but I wrote "leading party figure" -- Eugene McCarthy was by any definition a marginal figure before he shocked the party by mobilizing anti-war sentiment early in '68. Of course, Bobby Kennedy then got into the act, but IIRC that was after it was already known (or becoming clear) that LBJ was withdrawing.
What Teddy Kennedy did in '80 did have some precedent in the '68 "insurgency" against LBJ, but I'd still say it was pretty shocking for the Demagogues that Teddy the Swimmer would help to weaken their incumbent and so pave the way for the great Ronald Reagan (Carter was vulnerable, to be sure, but I think that Teddy's attack from the left helped to weaken Carter's standing even more).