This would not have been hard to set up as the radical republicans had plenty of foreknowledge that the southerners were not going to vote for Lincoln, and that if their dream of electing Lincoln was to come about it would have to be in the Northern states. One of the things often overlooked in the politics of the late 1850's is that the Republican party was not a "new" political party, but simply a renaming and rebranding of the old Whig party. Therefore it had the same political inferstructure in place as with the Whig party and the same political operatives. One of Lincoln's closest political advisors was the Presidental candidate of the old Whig party in 1852, General Winfield Scott.
As for the 1864 vote I had read somewhere that the voting process in the Union army was unregulated and that you could vote as many times as you pleased since the vote wasn't taken on a regimental level and the vote wasn't compared to any muster rolls.
Interestingly enough the Democrat candidate who recieved the most popular votes but the least electorial votes was Stephen Douglas. Douglas died early in June 1861. Had Douglas been elected as president his early death would have elevated his Vice presidental candidate Herchel Johnson from GEORGIA as President in 1861.