Let's say the Civil War goes just a bit better for the Union; for a specific POD say Grant avoids Cold Harbor due to some better intelligence about what he would face, and Grant crosses the James a couple weeks ahead of schedule. Sheridan is successful at cutting the Virginia railway lines into Richmond, though with even heavier losses (let's add some drama to it by making Sheridan one of numerous generals wounded in the war), and the Battle of the Crater therefore succeeds due to butterflies putting different USCT there under a different commander. Richmond falls in later summer, and while the CSA government can more easily flee, and Lee retreats with them to Danville for a time, he finally surrenders before the election.
Now, Cold Harbor damaged Grant's reputation OTL and made it appear to Lincoln and others in his party harder to win re-election (whether it was is for a different thread). So, you could argue that maybe TTL they pick Hamlin instead of Johnson. But, the point is, Lee's out by October, having surrendered the last of his army after failing to defend his home state, but there's enough life in the CSA that John WIlkes Booth forms his conspiracy to kill Lincoln and Hamlin just before the election, thereby throwing the Union into as much turmoil as possible.
So, what happens if Lincoln and Hamlin are killed on, say, November 1, 1864 in this scenario? (Obviously it'd be more damaging if he tried OTL, but since he apparently wasn't trying to do so OTL but just talking about it and fuming inside, it seems much more plausible for him to do so after the Union has for all intense and purposes won)