MLK was mostly non-partisan, from what I've read. I know he worked closely with LBJ and was about as far-left liberal in his ideologies as one can get.
Here's a good article I came across about this:
John R. Bohrer: Dr. King: A Registered Republican? - Politics on The Huffington Post
To say that MLK was a conservative or a Republican is a little like saying that Reagan was a fiscal and social liberal. The facts just don't support the notion.