In The Private History of a Campaign that Failed Twain recounts his risible participation in an informal local militia formed to fight for the Confederacy. He was lucky not to have been hanged by a Union colonel named Grant who was rounding up and hanging little guerrilla bands like his. Without benefit of an ACLU Lawyer. It speaks well of his character that he provided financial support for Grant at the end of his life.
In his writings he is extremely sympathetic to Blacks and clearly feels that their talents and strengths are under appreciated, but I don't think he could have been called an abolitionist in the sense of publicly advocating the end of slavery prior to the Emancipation Proclamation.
I stand corrected.
I think that excerp was actually posted by someone else, and not the part of the post I was referring to. Sorry for the mistake.