Why was the election of 1864 notable?  

The presidential election of 1864 was extremely important in American history. In this race, Abraham Lincoln defeats George B. McClellan. With this win Southern states were worried about the future of slavery, as they felt that the newly elected president Abe Lincoln would try to get rid of this institution.

As a result, South Carolina secedes (aka leaves) the United States only one month after Lincoln wins the election. Within a few months of that, several other states follow. Essentially, the election of 1864 resulted in the creation of the Confederate States of America and the beginning of the American Civil War.