I think that Humphrey ran a very strong campaign given how weak he was coming out of the convention. He did an excellent job of clawing back WWC voters in the north who might have potentially gone over to Wallace and also managed to win Maine which at that time was a fairly impressive feat for a Democrat given that it's something FDR wasn't able to pull off once and Johnson only did in the Goldwater landslide. Meanwhile Nixon ran an antiseptic stage managed campaign and probably came off as a coward for his refusal to debate. If HHH had had just another week he probably would have won easily.
The main reason he won ME - or only reason I should say - was Muskie. Don't think he would have otherwise.
He did win by a more comfortable margin then you’d expect though if Muskie was the only factor. I think that rural New England trended left significantly between 1960 and 68. Look at how much Humphrey outperformed Kennedy in 60 in even Massachusetts and I think that it might be the sign of a regional trend that bleeds over into Maine.
