What about talking more about the good economy? I'm sure the economy mattered to many voters in 1968. If Humphrey would have talked more about JFK's and LBJ's economic record and about how he'll continue it, I'm pretty sure he would have gotten some voters that were unhappy with Vietnam due to their desire to keep the prosperity going.
Or he could have just opposed the Vietnam War entirely.
Didn't he kinda do that on the campaign, though? He said he'd end the war as soon as possible.
Not really. If I recall, at the end of the campaign, he only inched a tad to the left of LBJ. He mostly ignored the Vietnam War entirely, which was stupid. If he was a critic of Johnson's Vietnam policy, he would have gained a large amount of support, and probably not losing much, if any at all either. Anti-War-ism wasn't exclusive among the new left, quite the contrary, it was a middle class phenomenon as well.