Forgive me here; I haven’t had any classes on this in ten years.
The official Union stance was that, since secession is illegal, the Southern states never actually left the US. (Again, this might be wrong or oversimplified. Let me know.)
So, if some clever individual had started a movement to get the southern states to send electors to Washington, the Union would have had no right to keep those electors from casting their votes for president. McClellan was running against Lincoln on an anti-war platform. Would the Confederate electors’ votes have counted? Would it have been unconstitutional not to count them? Would it have made any difference?