If Johnston or (less likely) Hood is able to inflict a severe check upon Sherman that prevents him from capturing Atlanta by Election Day, it is likely that Lincoln will lose the election and McClellan will be the next President. Lincoln himself thought so, as demonstrated by his memo on the subject written in early August.
In contrast to most people on this board, I think a McClellan electoral victory at least might have resulted in Confederate independence. If the Confederates are able to avoid a severe defeat before the end of the 1864 campaign season (which would have to be the case in any TL involving a McClellan electoral victory), they will begin 1865 in a vastly improved position than was the case IOTL. There would have been no March To The Sea and possibly no ravaging of the Shenandoah Valley.
The Northern public appeared quite willing to throw in the towel during the summer of 1864, until the victories at Mobile Bay, Atlanta and in the Shenandoah caused public opinion to swing back towards Lincoln. The prospect of 1865 being yet another bloody and indecisive year could have been too much for a tired Northern public, which would have already demonstrated its reluctance to go on with the war by electing McClellan.
Furthermore, as Lincoln clearly understood, McClellan would have owed the anti-war wing of the Democratic Party too much to be able to resist the political pressure to enter into a cease-fire with the Confederacy. This was seen during the election itself when the Democrats nominated an extreme antiwar Copperhead as its VP candidate. Although McClellan himself would have probably gone into any such negotiations with a good faith intention of restoring the Union, there was no way the Confederates would have accepted anything other than complete independence. And once the fighting had been brought to a halt, it would have been physically impossible to resume hostilities no matter what the outcome of the negotiations would have been.
Finally, it seems extremely unlikely that the Republicans, and in particular the abolitionists, would have been willing to continue prosecuting the war effort with McClellan as commander-in-chief. If McClellan had dropped emancipation as a condition of peace, which he certainly would have done, the Republicans would no longer have seen the war as worth winning. Indeed, I would expect at least half of the U.S.C.T. units to mutiny and throw down their weapons the moment President McClellan publicly drops emancipation as a Union war aim.