MLK was not a Republican, and you ought to read about the bad things he said about Barry Goldwater in the 1964 election. It sounds like the sort of rhetoric you’d hear from Al Sharpton today.
I stand corrected. MLK’s son says his father never endorsed any candidate and that he was likely not a Republican.
Here’s SNOPES take on MLK:
http://www.snopes.com/history/american/mlking.asp