And all this is predicated on Grant being stopped long enough for a victory at the polls for the Democrats in 1864?
Problem 1:
Sherman has still taken Atlanta, and is Marching Through Georgia. At best for the Confederacy, a nastier campaign in front of Petersburg will only lessen Lincoln's OTL landslide. Neither Hood nor Johnston would have been able to stop Sherman. The only difference being that with Johnston in charge Sherman's advance would have been slower. But Johnston staying in command would have required Jefferson Davis to not be the man he was.
Problem 2:
Assuming George B. McClellan wins, he still has 119 days until he is sworn in as Commander-In-Chief. By Inauguration Day March 4th 1865, Georgia has fallen, South Carolina has fallen, and North Carolina is about to be invaded. Mobile has been secured, Arkansas has fallen, Tennessee has been secured, and the Union is preparing to march across the rest of Mississippi and to invade the interior of Alabama. Only Texas, most of the Indian Territory, and the hinterlands of Florida, southwest Georgia, and (pro-Union!) Appalachian parts of North Carolina and a rump of Virginia are left.
Now, you are President George B. McClellan. The grounds are drying, and the new campaign season in Virginia is about to begin. Maybe the Democrats have control of the US House of Representatives, maybe not. But they certainly won't have the Senate. So, what do you do? Your Copperhead Democrats are demanding PEACE NOW. But you are a War Democrat, as are any number of your supporters. And whatever the power Copperheads had within the Democratic Party following a victorious 1864 election, what would they have left in a USA that had the CSA not just by the throat but prostrate and helpless?
Do you as President McClellan suddenly run off and sign a peace treaty with the CSA, recognizing their Independence, and thereby signing away potentially a massive portion of your own power as POTUS over ALL of the United States, including the about to be defeated Southerners (Democratic voters all)? Especially considering that the chances of a US Senate (still dominated by Republicans) giving a 2/3rds approval of such a peace treaty are just about nil?
No.
He'll happily shred (or let the courts shred) the Emancipation Declaration, and let the 13th Amendment die (while letting the 14th and 15th stay stillborn). But only as a condition for the South's surrender. Even then, probably an "unofficial condition", since he'll like getting that Unconditional Surrender for himself, just to spite Lincoln. Though no doubt with his opinions about Blacks being no secret, the Southrons will certainly believe his reassurances.