American Quebec?
If they did, how would this affect World War II? Might Hitler win?
/sarcasm
If the US has Canada, that's likely to shift the balance of power in favor of the Northern Free States much earlier, and might lead to an earlier civil war that the South has a better chance of winning since Northern industrial might hasn't developed as much yet.
But the Civil War was not about states' rights. If Quebec was going to secede, it might ally with the Confederacy, but for completely different reasons.