Very impressive letter from Lincoln. A few excerpts:

"At the last session of Congress a proposed amendment of the Constitution, abolishing slavery throughout the United States, passed the Senate, but failed for lack of the requisite two thirds vote in the House of Representatives. Although the present is the same Congress, and nearly the same members, and without questioning the wisdom or patriotism of those who stood in opposition, I venture to recommend the reconsideration and passage of the measure at the present session."

(sounds like simple persistence made the difference here)

"I repeat the declaration made a year ago, that 'while I remain in my present position I shall not attempt to retract or modify the emancipation proclamation, nor shall I return to slavery any person who is free by the terms of that proclamation, or by any of the acts of Congress.' If the people should, by whatever mode or means, make it an Executive duty to re-enslave such persons, another, and not I, must be their instrument to perform it."

Abolition of Slavery