Why Lincoln won election of 1864?

It wasn't so much that Lincoln was unpopular, but he was committed to the war, and the war seemed hopeless until September 1864. Three years of bloodletting saw the Union seemingly no closer to victory than when it began. When the military campaigning began in 1864, when the weather was warm enough and the dirt roads dry enough for the armies to move, about the second week in May, the most horrific losses yet began. Many newspapers in the north published the list of names each day of those killed, and there were very, very long lists, every day. Sherman was operating in Georgia, and Grant was now operating against Lee in Virginia. In six weeks Grant lost more men than Lee had in his army, and still he pushed on. But to the average civilian eye nothing much seemed to be accomplished to compensate for this carnival of death. Grant was still not in Richmond, and Sherman was still not in Atlanta. All summer long this went on. In late August Lincoln himself thought he would lose the election come November. He had the cabinet secretaries sign a paper, folded so they could not read it. It was a pledge to try to help Lincoln get the war won between the election and the swearing in of the next new president, which in those days was done on March 4 the following year.

Then something miraculous happened. Sherman captured Atlanta. Nothing was more important in securing the election for Lincoln, and thus for winning the war for the north.

Lincoln's opponent was General McClellan, whom Lincoln had basically sent home after the Battle of Antietam and the mid term elections of 1862. McClellan had been without a command since. Those in the know were aware that McClellan had done a poor job as a general, and had blamed Lincoln for his own shortcomings. There was something slightly nauseating about a general trying to unseat the president in the midst of war. McClellan's platform was making an end to the war, which would have meant that the south had won, had gained what they were fighting for, independence. Many people were reluctant to see that happen, after all that had been sacrificed in the war, even before Atlanta fell. The soldiers of the Army of the Potomac had loved McClellan when he was their commander, for making an army out of them. But they voted overwhelmingly for Lincoln, so that their sacrifices and the friends they had lost would not be for nothing. Many states did not have absentee balloting then, so Stanton, the Secretary of War, furloughed entire regiments and made sure they had trains available to get them back home, just so they could vote.