Let's say McClellan wins, but Lincoln somehow pulls off getting a Confederate loss and hands him reconstruction. The South would have their representation in the federal Congress restored, probably after some kind of loyalty oath of a percentage of the population. If McClellan wins, the Republicans will likely have lower numbers in both chambers, or be in the minority, meaning the radical reconstruction - overthrow the governments that just passed the 13th amendment and throw them out of congress till they pass the 14th amendment - won't happen.
So, McClellan wins, the 13th passes to outlaw slavery. He allows those same state governments to take oaths to the Union and those governments, in continuity with the government running the south 1861-1865, doesn't pass the OTL 14th or 15th amendments. At some point soon, the Supreme Court will rule that the freed slaves are citizens, and that could trigger an alternate 14th amendment: "All persons formerly held to service or labor, born within the United States, and under the jurisdiction thereof, are citizens of the United States, and of the state wherein they reside." If worded this way, that avoids later re-interpretation to widespread birthright citizenship for illegal aliens and avoids birth tourism by foreigners in the US. But that's a later topic.
This new timeline, slavery is gone, thank God, but equality is not going on just yet. Without reconstruction there is likely no KKK, so less racial strife overall. If the Democrats of this time period retake all chambers, they may stifle black civil rights and legal equality at a federal level. Let's say also that the federal government redirects some tariff revenue south towards rebuilding infrastructure, rather than leaving the south destitute for decades, since McClellan has some pull with Congress. Railroads get rebuilt quicker; livestock is purchased and replaces that stolen, killed, or eaten by Union troops; homes and businesses are rebuilt sooner; blacks are employed doing this as are whites. Without the radical Republican Congress it's very likely the south would've recovered from Sherman's devastation much quicker and the south as a whole would have come out ahead decades earlier. But that's if Congress diverts funding to the south for rebuilding, like we did with the Marshall Plan for Europe. The South had no money, no credit, no livestock, no crops or seed, and many were homeless after the war.