It turns out that Lincoln had actually come late to the realization about a need for the anti-slavery Thirteenth Amendment (not until June 1864).

FORBES: In Lincoln (2012 Movie), Why Did Lincoln Rush The Vote On The 13th Amendment?

Congressional passage of the Thirteenth Amendment in January 1865 was nonetheless a stirring, profound moment in American history, but it was not the stark turning point that the movie pretends it to be.

FORBES: In Lincoln (2012 Movie), Why Did Lincoln Rush The Vote On The 13th Amendment?

Real life is fascinating, and the real history of the Thirteenth Amendment is complex and compelling, but nobody should deceive himself into imagining that the full history of three-year constitutional battle can be turned into a two-and-a-half hour film that gets your heart pounding and fills your eyes with tears as this wonderful story does.

FORBES: In Lincoln (2012 Movie), Why Did Lincoln Rush The Vote On The 13th Amendment?