IRL
April 1863
In his second tome on the Civil War, Shelby Foote relates one of Lincoln's visits to the front, while the Army of the Potomac was camped on the Rappahannock near Fredericksburg. He is taken to a hill to look across the river at the town, which was still held by the Confederates. They are within shouting distance of the other bank, and seeing people looking across, the CSA sentries on the other side start taunting them. A Confederate officer comes to see who they are yelling at and takes his binoculars, realizes who it is on the other bank and removes his hat, makes a showy bow towards the Union watchers, and then walks away.
POD
Having been at the very forefront of the lines, sighted by the enemy, and if instead of a gallant officer seeing them, a Confederate sniper armed which a Whitworth rifle had been the one to spot the important, and quite targetable president. And having successfully landed a fatal shot, what would be the likely repercussions, and inevitable impact on the war.