Possibly as badly as Ike's contempt for Mr Nixon hurt the latter in 1960. My memory is that many regarded Mr Humphrey as spineless, fairly or not. And whatever Mr Nixon's flaws -- and they were legion -- spinelessness was not perceived as one of them. Opportunistic weasel, yes. And while in the Democratic party, being "tough," particularly on Communism, was not as big a deal in 1968 as it might have been before Vietnam, I wonder how much of the country at large in 1968 valued the perception of "toughness," especially in light of all the damned hippies running around protesting and doing drugs. The GOP floated the argument that the U.S. was a much more conservative country than appeared in 1968 (and especially in 1972 et seq), and managed to make it fly, particularly by appealing to the bigotry and fear of what should have been solid Democratic voters. (Why is it that the U.S. is one of the few places where the working class is not inclined to Socialism?)
Which I will use as a segue to another question: the soi-disant "Socialism" of Mr Sanders. There are those in the GOP (to say nothing of the Democratic party), who believe that this taint will hurt Mr Sanders in an election. To the point where there are those in the GOP who would apparently prefer to run against Mr Sanders than Mrs Clinton. I am unconvinced that this will be as important a factor as these pols believe, yet I am humble enough to acknowledge that those who do politics for a living may just know more about their field of expertise than I. But here are my thoughts: as for the generation which flourished before the destruction of the Evil Empire, the socialist bogeyman probably has great potency. We grew up in the Cold War, and were force-fed the Kool-aid on a constant basis by even the most "liberal" of politicians. But since the breakup of the USSR, and the transition of the PRC from an agrarian paradise to a capitalist's wet dream, the generations since have little or no reason to fear socialism or regard it with suspicion: it is just a word to them, not the embodiment of Satan it was for earlier generations. (Of course, regional and religious caveats apply) So one of the more interesting questions of the ongoing race will be to see how great an influence the Socialist label has on the results. I am inclined to believe the Red-baiters are backing the wrong horse, but again, I am not an experienced politco.
-- Mal