In the movie Lincoln Why does president Lincoln not want the southern peace negotiators to come to Washington dc?

In the movie "Lincoln," President Lincoln is hesitant to allow the southern peace negotiators to come to Washington, D.C., because he fears it might legitimize the Confederacy and suggest that they have equal standing with the Union. He is focused on passing the Thirteenth Amendment to abolish slavery, believing that a strong legislative victory would undermine the Confederates' bargaining power. Lincoln wants to ensure that any negotiations are framed within the context of Union victory and the end of slavery, rather than a peaceful reconciliation that could allow for the continuation of the status quo.