Found this on MLK was a Republican:
http://www.washingtonpost.com/wp-dyn/content/article/2006/10/18/AR2006101801754.html
I should find you the discussion on this on FR, but MLK, Jr. was never officially affiliated with either party, but he was effectively a left-wing Democrat by the 1960s. MLK, Sr. WAS a Republican but un-endorsed Nixon and supported JFK in 1960 when the Kennedys reached out to MLK, Jr. after his arrest, but was a Democrat from that point onward. MLK, Jr. favored Big Government solutions to the problems of the Black Community.