when Humphrey entered Democratic primary in 1960 he was viewed as very very liberal and had a strong liberal fanbase which was what hurt Kennedy and almost lost him the nomination.
HHH entered too late to be a factor in the primaries. He was seen as the party regular compared to the "peace" candidates RFK and McCarthy. HHH got many of the delegates from party appointments not from primaries, though many RFK delegates moved to HHH after the assassination. It is often overlooked that McGovern entered the 68 race after the assassination (not just in 72) as yet another peace candidate.
I think that marks HHH as a comparative moderate in the eyes of voters in 68. No doubt his social agenda was liberal, but the war was the litmus test at that point in history.
Like most presidential elections, 68 was a referendum on the party in power, and RFK could not overcome the negatives for Democrats that year. If he survived he would have been a formidable candidate in 72 or 76.