Final answer:
The capture of Atlanta was pivotal for the Union's morale and Abraham Lincoln's re-election in 1864. Had Atlanta not fallen, public support for Lincoln could have diminished, increasing McClellan's chances of winning the presidency.
Step-by-step explanation:
The statement that Lincoln might have lost the presidency to McClellan if the Union had not taken Atlanta in September 1864 is true. The military victories of the Union, particularly the fall of Atlanta, significantly boosted Lincoln's popularity. The success in Atlanta was not merely a military achievement; it was a psychological boost for the North, which had grown weary of the prolonged conflict. The victory helped to ensure Lincoln's re-election, as it validated his war strategy and undermined McClellan's platform, which was peace negotiations with the South. This was evident in the November 1864 elections where Lincoln secured a vast majority both in popular votes and electoral votes.
Without the fall of Atlanta and subsequent military successes, the public's support for the war, and consequently for Lincoln, could have waned, opening the door for McClellan, whose Democratic Party sought an immediate peace with the Confederacy, possibly at the cost of preserving the Union. The capture of Atlanta by Union forces led by General William Tecumseh Sherman was a turning point, leading to the eventual re-election of Abraham Lincoln as President of the United States.