Dumping Hannibal Hamlin for Andrew Johnson in 1864 had no significant impact on the election itself. Remember, the Vice Presidency at that point was still nearly a century away from becoming even somewhat relevant and more than 100 years from being truly influential. Switching running mates in 1864 had nothing to do with winning or losing. It was more of a PR move than anything.
Lincoln realized that the end of the Civil War was approaching and, after nearly four years of bloody war and an even longer period of sectional unrest, Lincoln was looking to the future. So, Lincoln, the Republican, looked to Andrew Johnson, a Democrat, to form a ticket that showed national unity. Sure, the choice of Johnson, a Southerner, helped balance the ticket more than Hamlin (who was from Maine), but Andrew Johnson wasn’t exactly a hero in the South. Many Southerners saw him as a traitor — he was the only Southern Senator who didn’t go along with his state (Tennessee in his case) when it seceded. Likenesses of Johnson was burned in effigy throughout the South, his hometown hung a banner in its streets that said “Andrew Johnson, Traitor”, and as Military Governor of Tennessee during the Civil War, Johnson was always well-armed — even while giving speeches — due to the scores of threats he received. If Lincoln wanted a Vice President that appealed to the vanquished South, Andrew Johnson wouldn’t have been much better of a choice than William Tecumseh Sherman.
President Lincoln wasn’t looking for geographical balance; he was seeking a political balance. Johnson was a Democrat that Lincoln felt he could trust, and that’s why the relatively (but very Republican) Vice President Hamlin was dumped from the ticket. But if Hamlin had remained on the ticket, I think the election would have still gone the same way. Elections in the 19th Century weren’t influenced by Vice Presidential picks.
Would Lincoln have chosen Johnson if he had a second chance? Assuming he didn’t know he was going to be assassinated, I think Lincoln would have stuck with the choice he made. Lincoln trusted his political instincts and, in most cases, they were remarkably good, so if he only thought that he was picking a Vice President, he would have stuck with Andrew Johnson.
But if Lincoln somehow knew that he was going to be shot then I highly doubt he would have wanted Andrew Johnson to succeed him. First of all, Lincoln was a loyal partisan. Choosing the Democrat, Johnson, for the boring, powerless, dead-end job of the Vice Presidency was no big deal. In 1864, the Vice Presidency sucked for four years and wasn’t even a stepping-stone to the Presidency at the end of the term, so Lincoln figured his act of national unity wasn’t doing any harm to the GOP because he was simply exiling a Democrat to the Vice Presidency.
If Lincoln somehow had the foresight to know that he was going to be assassinated, though, there’s no way he’d put a Democrat or Andrew Johnson in the position to succeed him (Then again, if he had that foresight, hopefully he’d skip Our American Cousin and read a book that night). Instead of Johnson, he probably would have kept Hannibal Hamlin as Vice President or possibly elevated William H. Seward to the job (even though moving from Secretary of State to Vice President was more of a demotion than elevation in 1864). But definitely not a Democrat.
But…some of you may note that Lincoln always had weird premonitions of his own death and that he had a desk drawer full of assassination threats and wonder why, with all of that, Lincoln would still pick a Vice President from the opposing party?
Well, Lincoln had weird premonitions and dreams all the time, so those were nothing new. The country was in the midst of a brutal war, so the threats weren’t surprising. Don’t forget — before Lincoln, no President had been assassinated and only one, Andrew Jackson in 1835, was the target of an unsuccessful attempt. Until John Wilkes Booth shot Lincoln at Ford’s Theatre, the assassination of the President wasn’t something that many people could imagine actually happening. Now, two Presidents had died in office by the time Lincoln was elected, but they were two of the three oldest Presidents elected up to that time (68-year-old William Henry Harrison and 65-year-old Zachary Taylor). At 56 years old, 6’4” Abraham Lincoln had led the country through a punishing war, but he was still in relatively solid health, had always been a naturally muscular and fit person, and he didn’t believe he would die in office and couldn’t imagine that someone would actually try to murder him. Had he believed that, there’s no way he would have picked a Democrat to succeed him, particularly in the important aftermath of the war as the country faced Reconstruction.