Honestly I just sometimes feel this need to counter this idea that the US no matter how banged up it gets or how much territory it loses will always be this big strong nation that'll settle the score down the line. It's entirely possible that if McClennan surrenders that he's unable to effectively repair the nation and leaves the presidency with the nation barely recovering and a mess of a political landscape for the next guy to try and fix up.Also true! The US came dangerously close to doing so more than once, the Civil War merely being a prime example of how there was an unstable political balance that knocked the US on its heels between 1861 and (arguably) 1876 forcing it to focus inwards and see quite a bit of political dysfunction despite the moral triumph of freeing the slaves and building the transcontinental-railroad.
The bitter feelings of a lost civil war (or even an uneven treaty) can have some less than salutary effects on a nation. There's a reason that in WiF the post-war period is known as "The Era of Hard Feelings" as opposed to the post 1815 "Era of Good Feelings" which defined the lead up to the sectional crisis.
A worse possibility is an embittered Union soldier catches him in the wrong place and the wrong time and assassinates him leaving the nation completely rudderless in the wake of this disastrous war.
America certainly has it's share of little Napoleon's who are willing to use that kind of chaos to advance themselves and their power and prestige at the cost of their nation.