Frederick J. Blue, "Sumner, Charles," American National Biography Online, February 2000, http://www.anb.org/articles/04/04-00969.html.
For Sumner the Civil War presented the opportunity to free the slaves, and he became one of the first members of Congress to urge abolition. He worked for the next eighteen months to persuade President Abraham Lincoln. During that time he skillfully pushed legislation that weakened slavery in numerous small ways, as he successfully prepared public opinion to accept black freedom. Clearly he was among the most important of those who influenced Lincoln to issue his Emancipation Proclamation. So, too, he helped convince Lincoln of the wisdom and justice of allowing blacks to join Union armies against the Confederacy in behalf of their own freedom.