IMO Maj. Gen. William Tecumseh Sherman would have had to have been assassinated in December of 1863 or January 1864...Then the south would have had a shot.
You’re absolutely correct. Sherman taking Atlanta in September of 1864 guaranteed the election for Lincoln.
Before that, Lincoln was almost resigned to the fact that he would lose reelection. Without that victory, the democrats (why is it always them giving up to enemies?) would have won and McClellan wanted to have a peace conference with the Confederacy. Now, Mac said the conference would be on what to do to get the Southern states back into the Union, but what would he have done if they refused; restart the war?