I believe there were two equally important moments determinative of the outcome of the Civil War. The first was Lincoln’s issuance of the Emancipation Proclamation on January 1, 1863 – which amounted to a nearly final guarantee the European powers would refrain from entering the war in league with the Confederacy. It also quickly led to an exponential increase in the contribution of African Americans North and South to the Union military effort. Approximately 180,000 black soldiers eventually served in the Union army during the Civil War, the overwhelming number of them enrolled after the Emancipation Proclamation. Moreover, almost 30,000 blacks served in the Union Navy, greatly enhancing the manpower of the North’s power at sea.If anything, May is probably understating the risks a McClellan Presidency would have brought to the Union cause.
The second of these moments was the point when Lee’s decision to invade Pennsylvania, made a few months after the Proclamation, became irreversible. The ensuing battle of Gettysburg was a catastrophic blow to Confederate manpower and morale. Had Lee remained in Virginia and adopted a more Fabian strategy in 1863, he could perhaps have avoided disastrous defeats, strung out the war, and induced enough war weariness in the North as to undermine Lincoln’s campaign for reelection in 1864. It is well known that Lincoln’s reelection in 1864 was uncertain through most of the the summer of 1864, even with the Union victories at Gettysburg and Vicksburg the year before. Although the Democratic candidate in 1864, General George McClellan, refused to run on his party’s peace platform, it is unclear whether he would have had the will to crush the Confederacy if elected. One has to wonder, given his longstanding reluctance to risk high casualties in battle and his deep qualms about both emancipation and taking the war to civilians, whether a President McClellan would have unleashed Grant and Sherman in 1864 the way Lincoln did. Without aggressive Union military strategies in 1864-1865, the Confederacy might have hung on long enough to eke out a fragile independence. [my emphasis]
Although it's worth remembering that Presidents then took office in March following the election, not in January as it is now. In the history that actually happened, Lee's surrender at Appomattox occurred in April. So the Lincoln Administration could very well have done such irreparable damage to the Confederacy by March 1865 that even a President McClellan sympathetic to the South might not have been willing to let the Confederate states go from the Union.