General George McClellan was Lincoln's Democratic opponent in the 1864 election?

Yes - and looking set to win until Sherman and Sheridan came in with some timely victories on land, and Admiral Farragut liberating the port of Mobile.

People still debate what would have happened if McClellan had won. He had distanced himself from the anti-war faction in the Democratic Party. But there would probably have been a compromise peace, and that would have meant some form of Southern independence.