He was an independent, who never publicly claimed to be for one party or the other.
Martin Luther King, Jr. Was he a Republican?
Martin Luther King Jr being a Republican or not, was already covered in a previous post. the sheer number of posts I have seen on Facebook in the last few days made it seem like a more indepth post on just this subject was a good idea. I have seen a few articles claiming he was a Republican, but they offer no proof at all, just speculation that because he was a black man in the south in the 1950’s and 1960’s he must have been a Republican. His father, Martin Luther King was a Republican.
In 2009, the National Black Republican Association and the Raging Elephant groups started advertising campaigns claiming that Martin Luther King Jr, was a republican. For the most part their proof was simple speculation that because he was black, and lived in the 1950’s and 1960’s, he had to be a Republican. They also derived their proof from his Niece Alveda King had also been making the claim that he was Republican. No voter registration exists, and no voting record other than what he himself was public about exists, so the only proof anyone has to go on is his own writings, and the comments by his family and the King Center of Atlanta.
So let’s go back to Alveda, since she is the reason the others say he was a Republican, and was the most vocal about it. In 2009 she said :
People ask me all the time, “was Martin Luther King a Republican?” In conversation, I once said that my uncle, Dr. King was a Republican. What I really meant, and should have clearly stated was that while I never saw his registration card, I firmly believe that Dr. King, like most Negroes (that is what African Americans were called during Dr. King’s lifetime) during Dr. King’s lifetime probably voted on the Republican ticket out of respect to the Party of Lincoln.
But, in 2013 she tweeted :
And posted on her blog :
I have few regrets in my life. At the top of the list is the demise of two children in my womb, and one miscarriage. Next to that, I regret having said to a group of peers that my Uncle M. L. (Dr. Martin Luther King, Jr.) was a Republican. I said that without having all the facts. My Grandfather, Dr. Martin Luther King, Sr. was a registered Republican. Uncle M. L. was an independent, who in his own words tended to vote Democrat. I assumed that since Granddaddy was a Republican, Uncle M. L. was too.
So her second most regretful thing in her life, next to losing children, was mistakenly claiming that Martin Luther King, Jr was a Republican. Sounds pretty serious. Serious enough that this claim alone should be proof enough that he was not a Republican, but an Independent. There is more though, so let’s continue.
February 11, 1958, during an interview at Bennett College, Martin Luther King, Jr said :
I’m not here to tell you how to vote, ”he said. “That isn’t my concern. I’m not a politician. I have no political ambitions. I don’t think the Republican party is a party full of the almighty God nor is the Democratic party. They both have weaknesses. And I’m not inextricably bound to either party. I’m not concerned about telling you what party to vote for. But what I’m saying is this, that we must gain the ballot and use it wisely
Sure sounds like he isn’t a Republican, in his own words. He had a few more things to say about it as well, he wrote in his Autobiography, in 1964 :
The Republican Party geared its appeal and program to racism, reaction, and extremism. All people of goodwill viewed with alarm and concern the frenzied wedding at the Cow Palace of the KKK with the radical right. The “best man” at this ceremony was a senator whose voting record, philosophy, and program were anathema to all the hard-won achievements of the past decade.
It was both unfortunate and disastrous that the Republican Party nominated Barry Goldwater as its candidate for President of the United States. …. On the urgent issue of civil rights, Senator Goldwater represented a philosophy that was morally indefensible and socially suicidal. While not himself a racist, Mr. Goldwater articulated a philosophy which gave aid and comfort to the racist. His candidacy and philosophy would serve as an umbrella under which extremists of all stripes would stand. In the light of these facts and because of my love for America, I had no alternative but to urge every Negro and white person of goodwill to vote against Mr. Goldwater and to withdraw support from any Republican candidate that did not publicly disassociate himself from Senator Goldwater and his philosophy.
On October 30, 1956, in a letter responding to Earl Kennedy, in which he asked King about his position “relative to Negro people voting for Democratic candidates nationally.” King replied 
I am not taking any public position in this election. In private opinion I find something to be desired from both parties. The Negro has been betrayed by both the Democratic and Republican Party. The Democrats have betrayed us by capitulating to the whims and caprices of the southern dixiecrats. The Republicans have betrayed us by capitulating to the blatent hypocrisy of conservative right wing northerners. This coalition of southern dixiecrats and right wing northern Republicans defeats every move toward liberal legislation in Congress. So we confront the problem of choosing the lesser of two evils. At this point I might say however, that I feel that the Negro must remain an independent voter, not becoming unduly tied to either party. He should seek to vote for the party which is more concerned with the welfare of all the people.
He also made similar comments at the “Facing the Challenge of a New Age,” Address Delivered at the First Annual Institute on Nonviolence and Social Change on December 3rd, 1956 and the “Desegregation and the Future,” Address Delivered at the Annual Luncheon of the National Committee for Rural Schools on December 15, 1956 
On October 27, 1960, in an interview after being released from the state prison in Reidsville, GA King made the following statement (and of note, after this interview, King’s father officially endorsed John F. Kennedy Jr for President) 
Well, I would not like to make a public statement concerning the person for whom I will vote because I follow a non-partisan course and heading a non-partisan organization, namely the Southern Christian Leadership Conference
In the July 12, 2008 edition of the National Journal Magazine, Steve Klein, the communications director for the King Center in Atlanta (started by Coretta Scott King) said that he wasn’t a republican, and didn’t endorse any party :
There is absolutely no confirmation that he was a Republican, He was never a member of any political party–and never formally endorsed any candidates.
In his autobiography, he made mention of his past comment about not endorsing any party, saying if he had known before as much about Kennedy, he would have endorsed him :
I had to conclude that the then known facts about Kennedy were not adequate to make an unqualified judgment in his favor. I do feel that, as any man, he grew a great deal. After he became president I thought we really saw two Kennedy’s -a Kennedy the first two years and another Kennedy emerging in 1963. He was getting ready to throw off political considerations and see the real moral issues. Had President Kennedy lived, I would probably have endorsed him in 1964. But, back at that time, I concluded that there was something to be desired in both candidates.
His son, Martin Luther King III said :
It is disingenuous to imply that my father was a Republican. He never endorsed any presidential candidate, and there is certainly no evidence that he ever even voted for a Republican. It is even more outrageous to suggest that he would support the Republican Party of today, which has spent so much time and effort trying to suppress African American votes in Florida and many other states.
As far as him being a “conservative” King himself had this to say  :
I imagine you already know that I am much more socialistic in my economic theory than capitalistic.
there must be a better distribution of wealth and maybe America must move toward a democratic socialism. Call it what you may, call it democracy, or call it democratic socialism, but there must be a better distribution of wealth within this country for all of God’s children.
 The Associated Press State & Local Wire July 4, 2008
 http://www.thekingcenter.org/archive/document/mlk-speech-sclc-staff-retreat (pg 19)