Contrary to what has been stated by Republican party members, Martin Luther King, Jr. never declared that he was a Republican. In fact, he never identified with either political party and was not considered a Democrat either.
Martin Luther King, Jr. made a statement while being interviewed in 1958 specifically saying that he was not bound to any political party. He also made statements regarding his feelings on the Republican party in his autobiography, specifically talking about how the party could appeal to racism. Martin Luther King, Jr. was killed in an assassination in Memphis, Tennessee on April 4, 1968 at only 39 years old.