So, for McClellan to win, the CSA must do a lot better against Sherman in Georgia, and an impressive victory in the Virginia theater could only help McClellan's chances.
Lincoln's determination to fight harder if he lost in '64 will run up against Lee's discovery late in the war of the value of well-built field fortifications (e.g. Cold Harbor, Bloody Angle, etc.) The butcher's bill might prove unacceptable to a public that has just voted for peace, and Congress might try to intervene.
Question, what would happen if one John Wilkes Booth moved his plans up to just after the election, placing an unpopular Andrew Johnson in command after the public had tacitly rejected the Republican ticket. Would the Union Army fight harder or would Northern soldiers undergo the temptation to avoid being the last casualty in an unsuccessful war and begin deserting?