1. If the Democratic Party had won the 1864 election it would have been seen as a clear repudiation of Lincoln's platform of union with emancipation. The party platform included a call for armistice; McClellan himself wanted to continue the war until the Union was restored. He would not be calling the shots until March 1865, by which time it might be obvious that Federal victory was right around the corner. As he was not as concerned that slavery end he might have been able to hold together enough support to finish the war while holding out that carrot for the Confederates. In any case he would not be an advocate for "surrender."
2. Too big a topic but nothing ventured, nothing gained: McClellan the "slow"… McClellan the "cautious"… McClellan the paranoid, bold-talking but non-acting commander… McClellan the inventor of phantom numbers… these have become such entrenched memes that getting at the truth is hard. I lean toward defense of his overall military judgment, as I tend to do with most generals facing the problems of this war. (No one seems to mind a careful war leader when his/her son is in the ranks.) I think the picture we have is in large part because we view him through Lincoln's eyes - meaning through prevalent civilian amateur ideas about how wars work, and the pressure Lincoln was under to be seen to produce decisive results quickly. McClellan's preference for a well-trained army, his (in my opinion sound) argument for using Virginia's rivers against the Confederate defense rather than the politically favored overland approach, and his reluctance to continually explain himself can all be interpreted in the best or worst light; for many it's the worst - and, no, his bombastic correspondence with the Mrs revealing a streak of self-importance in his inner world doesn't help. Ultimately though McClellan is a War Democrat increasingly unable to work with a Republican administration. In the political arena, "deliberate" becomes "cowardly" or "traitorous" with great ease.
Others are likely to be along shortly with well-researched details about the pros and cons of his military decisions/actions on the ground…