When and why did the North become committed to emancipation (freeing the slaves) during the Civil War?

When Lincoln was inaugurated as President in January 1861, he declared that his first priority was preserving the Union, not ending slavery. When the war began a few months later, Lincoln echoed the same sentiment, as he knew many white Northerners would not be interested in fighting a battle for emancipation but would get behind the patriotic notion of fighting for the good of the country.
Lincoln even upheld the barring of African-Americans from serving in the army (though they had served in the Navy for many years) as a way to show Northerners that their "white privilege" was not being threatened by the war.
However, the shift towards considering emancipation began in August 1861 when escaped Southern slaves were declared as "contraband," since their labor was being used to aid the Confederacy. Those former slaves declared as such were then deemed to be "free." While they were still not allowed to serve in Union armies, they were at least free in the eyes of the law.
By early 1862, Lincoln was starting to entertain the idea of emancipation, as it became apparent that the presence of slaves was aiding the Southern war effort. With the North still not allowing African-American army volunteers, there were growing concerns about their ability to win the war. Additionally, many European governments were starting to recognize the South as an independent entity. Emancipation, which had already been enacted in much of the rest of the world, would sway foreign governments back to favoring the North.
In July 1862, Lincoln composed a draft of the Emancipation Proclamation for his Cabinet. Many government officials were still skeptical, but Lincoln persisted. He held off issuing it until the major victory at Antietam in September 1962.
While Lincoln personally felt emancipation was the morally correct path, he sold it to many as a military strategy. With slaves freed, they would no longer be pressed to help the Confederate war effort. Additionally, the North would open up enlistment to African-Americans and use their manpower to help win the war. Even for a few a years after the Proclamation, many Northerners remained skeptical. However, by 1865, many Northerners were starting to favor emancipation as Union victories started to mount up.
https://dcc.newberry.org/collections/lincoln-the-north-and-the-question-of-emancipation

https://www.abolitionseminar.org/how-did-americans-react-to-emancipation/