Lincoln as president of all of the United states of America was aware that the south had constitutional protection for its use of slavery.
Lincoln said" that slavery was a moral a social & a political wrong.
as the south was using there slaves to keep the home front going while the army was fighting,& using slaves in other ways to assist the war,
freeing the slaves would give the union forces an advantage.
Lincoln figured that constitutional protection of slavery could be overridden by sanctioned war powers of the presidency.
Lincolns main goal was to preserve the union of the UNITED states.
Constitutional protection of slavery? Perhaps in the sense that the Constitution didn't address it and left it up to the states to decide on an individual basis. Other than that, I have no idea what you're talking about.
The Emancipation Proclamation had NO effect on the Southern war effort, since it had no real ability to free anyone in territory not actually controlled by the Union . . . and it only applied to slaves in territories not controlled by the Union.
It was propaganda to make fighting the war a moral issue, because a big chunk of his troops were about to run out their terms of enlistment and go home.