During this period in American history, presidential candidates rarely campaigned for themselves, but rather had surrogates do it for them. It was seen as somewhat beneath the dignity of a person seeking such a high office to campaign on their own behalf. It was sort of like the modern equivalent of a person being nominated for a prestigious award like an Oscar or BAFTA: it's considered bad form to go on the talk show circuit, and talk up how amazing your performance was. Nowadays, the studio campaigns for you, and your co-stars might talk about how awesome you were. Now, obviously, this isn't a one-for-one comparison (actors still do the talk show circuit these days to build up buzz ahead of awards season), but you probably get the idea.

For the 1864 election, there were certain newspapers and editors who were known to sympathize and/or rally behind a candidate or party, and much of the election bad-mouthing was done through these organs. Famous public speakers, and cabinet members of the president would fan out in key states and cities to deliver speeches, etc., in support of their man, and this was what was expected at the time. These people did talk up McClellan's military ineffectiveness, yes, but the crux of their arguments against George relied mostly on his Copperhead sympathies, and the fact that he was a peace candidate.

[Sources: Doris Kearns Goodwin, 'Team of Rivals'; Bruce Catton, 'Glory Road']