Well there's still the problem of the Democrats having an incoherent platform, with different wings of the party wanting to dgo in much different directions. There were those Democrats who were outright sympathetic to the Confederacy and opposed the abolition of slavery, those that though they supported abolition believed that the Confederacy had a right to independence and those Copperheads that wanted to bring them treacherous Dixies back into the Union by whatever means it demanded so that they could thereafter hang Jeff Davis from the highest tree in Washington D. C. The Democratic ticket of 1864 had a pro-War Presidential Nominee (McClellan) an anti-War Vice-Presidential Nominee (Pendleton) while the party platform was for peace with the South (and personally rejected by McClellan). This contributed greatly to Lincoln's landslide: most people didn't know what they would be voted for if they voted Democratic. National Union on the other hand was clear and consistent in their program and nominee. Additionally, as a general, McClellan wasn't actually that good. Lincoln was at one point so irritated about McClellan that he remarked "If General McClellan does not want to use the army, I would like to borrow it for a time."
So, the timeline you're thinking about creating... I assume that McClellan will be a Peace Candidate, rather than the pro-War candidate he was in our timeline... Maybe, under the circumstances you're suggesting. If we just went with our timeline up to the election and still had McClellan winning, I would be offering the following reply:
"Just like John Quincy Adams appointed Henry Clay Secretary of State after the latter had helped the former win the Presidential Election of 1824, so George B. McClellan appointed Bmelmixibarotto Orglaxinicatzuki-Flephlminskiowraxxxa'thoplathlon, an Alien Space Bat from Alderamin VII, for his hand (or more aptly, wing) in letting the Democrats win the 1864 election, as Secretary of the Treasury."