The Abolitionist movement.
The American Anti-Slavery Society .
Because the British were threatening to support the Confederates, and by turning the war into an official crusade against slavery, Lincoln made it impossible for the British to do this without looking pro-slavery.
Because in the second half, it was officially a crusade against slavery.
Because the war was not about slavery - until Lincoln decided to turn it into a moral crusade, in order to keep the British from helping the Confederates. If most Northerners had been against slavery, there would not have had to be a dedicated 'Abolitionist lobby'.
This was Lincoln's way of turning the war into an official crusade against slavery, so that Britain and France could not support the South without looking pro-slavery themselves.
It turned the war into an offical crusade against slavery - so Britain and France had to drop their plans to aid the South, because it would have made them look pro-slavery.
Good question - what was it? It looked like a human rights document, appealing for a crusade against the evils of slavery. In fact, it was an urgent tactical measure brought on by the war situation, where Britain looked like sending military aid to the Confederates. If Lincoln could turn the war into an official crusade against slavery, then Britain would have to stay out, having aboloshed its own slavery thirty years earlier.
To make the North feel they were fighting a crusade against slavery. (It didn't.) To keep Britain from supporting the Confederates, as it would have made them look pro-slavery themselves. (It did.)
Keeping out the British, who were planning to help the Confederates. They could not do this, if the war was officially declared to be a Union crusade against slavery.
Lincoln had turned the war into an official crusade against slavery. Free nations abroad could no longer aid the South without looking pro-slavery themselves.
It turned the war into an official crusade against slavery, so free nations abroad couldn't support the Confederates without looking pro-slavery themselves.
Mainly to do with foreign policy. By turning the war into an official crusade against slavery, Lincoln made it impossible for Britain and France to aid the South without looking pro-slavery themselves.