McClellan ran on a platform based on ending the war in a truce with the Confederacy that allowed the south to return to the Union and an agreement the south could keep slavery until the end of the century. In which time, the U.S. Government would purchase their slaves and repatriate said slaves to a colony in Africa (Liberia).
At this time, a war weary North was ready to end the war and Union soldiers who had served under General George B. McClellan admired him greatly, so Union soldiers voted almost entirely in his favor. However, these Union soldiers votes were counted by Radical Republican Union Officers, and surprise, surprise the fraudulently counted votes of the Union Army threw the election to Lincoln.
Twelve states allowed absentee voting by soldiers. Lincoln got 119,754 to McClellan's 34,291. This was despite Democratic Party measures to prevent soldiers from voting. (Amazing how things stay the same, isn't it?)
Lincoln won the popular vote by over 500k, so obviously the soldier vote didn't change that result, though it might have made the difference in some states.
In particular, would you like to present something vaguely resembling evidence that Union Army officers counted the absentee votes, or at any rate counted them all? Here is some evidence that sealed ballots were mailed back to the home state and counted by officials there.
http://www.co.washington.ny.us/PDF/His/ABSENTEE%20BALLOTS%20FOR%20SOLDIERS1864.pdf
What do you got, besides a whine?