Lincoln was certainly acquainted with Keckley (and presumably with King Lear, whence “bare, forked animals” comes), but in the context of the times, he may have thought of black people as unknowable. At any rate the climate of opinion in 1865, even among progressive people in the North, was not such as to make racial equality an easy sell.
In fact, if the public got the notion the 13th Amendment was a step toward establishing black people as social equals, or even toward giving them the vote, the measure would have been doomed. That’s where Lincoln’s scene with Thaddeus Stevens comes in.
Stevens is the only white character in the movie who expressly holds it self-evident that every man is created equal. In debate, he vituperates with relish—You fatuous nincompoop, you unnatural noise!—at foes of the amendment. But one of those, Rep. Fernando Wood of New York, thinks he has outslicked Stevens. He has pressed him to state whether he believes the amendment’s true purpose is to establish black people as just as good as whites in all respects.
You can see Stevens itching to say, “Why yes, of course,” and then to snicker at the anti-amendment forces’ unrighteous outrage. But that would be playing into their hands; borderline yea-votes would be scared off. Instead he says, well, the purpose of the amendment—
And looks up into the gallery, where Mrs. Lincoln sits with Mrs. Keckley. The first lady has become a fan of the amendment, but not of literal equality, nor certainly of Stevens, whom she sees as a demented radical.
The purpose of the amendment, he says again, is—equality before the law. And nowhere else.
Mary is delighted; Keckley stiffens and goes outside. (She may be Mary’s confidante, but that doesn’t mean Mary is hers.) Stevens looks up and sees Mary alone. Mary smiles down at him. He smiles back, thinly. No “joyous, universal evergreen” in that exchange, but it will have to do.
Stevens has evidently taken Lincoln’s point about avoiding swamps. His radical allies are appalled. One asks whether he’s lost his soul; Stevens replies, mildly, that he just wants the amendment to pass. And to the accusation that there’s nothing he won’t say toward that end, he says: Seems not.
Later, after the amendment passes, Stevens pays semi-sardonic tribute to Lincoln, along the lines of something the congressman actually once said: that the greatest measure of the century “was passed by corruption, aided and abetted by the purest man in America.”