Wrong. If Atlanta hadn't fallen prior to the 1864 election, McClellan would have become President and sued for peace, meaning the South would have won.
I also believe that the North would have lost without Grant.
Had McClellan won he wouldn't have been inaugurated until March 1865. By that time the southern cause was all but over anyway. Besides, McClellan had repudiated the Democrat peace plank in the platform before being nominated.