The former star of "The West Wing" and "Studio 60 on the Sunset Strip" will play Abraham, "a powerful, well-dressed, sophisticated gay man who is a political activist," the cable network said in a news release.
"Shameless" stars William H. Macy as an alcoholic single father whose six children are frequently left to their own devices to survive. Emmy Rossum plays his oldest child, who does her best to care for her younger siblings.
Season 3 is to premiere Jan. 13.
Notable deaths of 2014 [PHOTOS]