You are incorrect.
Buchanan eventually lent his support to that amendment, but it was first and foremost a Lincoln project. William Seward first proposed it before the Committee of Thirteen in December, 1860 after recieving direct instructions from Lincoln to do so. From there on Lincoln carefully managed, and at first in great secrecy, its path through Congress up until the days before it was voted on by the full house and senate. Newspaper accounts at the time document extensively that Lincoln publicly lobbied for it during those final days. Henry Adams credits the amendment's success in Congress entirely to Lincoln, noting it passed due to the "direct influence of the new President." Lincoln followed a few days later by openly endorsing it in his inaugural address.
Pardon my ignorance and my perplexity, but if the (proposed) 13th Amendment was indeed a Lincoln project, then why on God's green earth didn't he come out earlier, before Dixie walked?
I had read briefly in Donald's Lincoln about this amendment, but I was concentrating more on the period 1854-1860, when he was formulating his national platform and deciding what to do about the slavery conundrum.