You're forgetting Southern diplomacy. If Lincoln loses, the South merely offers an armistice. Lincoln could refuse, but winter campaigns wont produce much. Chances of hostilities resuming are small. Sherman would have problems locating his army for resupply and garrison duty. The South gets a breather to reorganize armies, if ever needed.

Lincoln certainly refuses. Sherman march was Nov./Dec of 64. He later entered the Carolinas at the start of Feb. 65. If instead Atlanta and Mobile Bay held till after the election, Sherman begins his march in Dec. Since he was actually marching in Dec. and Feb., this would not be a problem. Wilmington fell in Jan., surely Mobile on the Gulf could be captured despite the weather. Hood destruction in Tennessee comes in Jan instead of very late Nov.But armies operated in Tennessee in Feb of 62 with McHenry and Donelson.

Hood destroyed, Atlanta gone, Georgia destroyed, Mobile and Wilmington shut down.Plus all that came before. Even McClellan would be able to see victory. In fact, McClellan would claim it was his despite the awful mess Lincoln left him.

Then would that ego of McClellan's allowed him to give up the presidency in 68 or later and leave a "lesser" man take over and ruined the Republic that McClellan "saved from destruction" three times.

I'm not sure Lincoln refuses the armistice. He would want to control the peace. Don't forget a McClellan win probably means democrats in congress. Lincoln would see a ticking clock. The allure of declaring victory before McClellan takes office would be intense.

Who is to say lee doesn't dig in, and Johnston avoids a certain loss like bentonville long enough for democrats to replace high command, end the draft, discharge troops, and present palatable peace terms? The South could have held out till 1866 by going asymmetric, longer depending on Texas and civilian will.

What is in it for McClellan politically? The north isn't staying democratic long.

With McClellan coming in, an armistance destroys the Union. What does an armistice gain the Union? Lincoln turns it down and orders the military to end the war quickly in victory.
Lincoln has five months. In March 0f 1865, Lee told Davies, Petersburg cannot be held and the capital will have to be abandon. With McClellan coming in, Grant turns up the heat and even with Lee trying to hold on, the same thing happens.

The war was won basically only on the strength of Northern industry and transportation (control of the railroads). Not to downplay the effect of the Union army or leadership, but, man-for-man and regiment-for-regiment they were really outmatched by the South (early on, at least) in terms of fighting ability, grit, experience, leadership and determination. Had the playing field been truly level (with respect to troop numbers, resources, infrastructure), the CSA wins, easy game.

grunching. Yes but not conventionally. They could have gone to the hills and fought a 10 year guerrilla war that would have culminated in the North going home and giving up. It would have been incredibly nasty and the end result would have been the south looking like a third world country today.