I read yesterday that the FBI knew that MLK was in some way involved with communist groups in America, but that information is still classified. Also, I've seen letters of his where he laments the "failures of capitalism" and talks about democratic socialism as being the best form of government. So, was he a communist? Democratic Socialist? How involved was he with communist parties?
There is no evidence to suggest that King was influenced by the Communist Party of the USA. The CPUSA claimed to it's KGB connections that it was able to influence King, but these claims were not believed by the KGB because of King's tendency to refer to the US Constitution and Declaration of Independence and to use phrases in his speaches like "we will win our freedom because the sacred heritage of of our nation and the eternal will of God are embodied in our echoing demands."
The KGB in fact attempted to discredit MLK (ineffectively with press attacks in some African newspapers) and replace him with a leader more willing to be aligned with International Communism. MLK holds the interesting distinction of having attempts to discredit him made by both the KGB and FBI.
The FBI did conduct surveillance on King, because one of his lawyers was thought to be a secret communist who had met with a KGB officer. There is some substance to this, but it is unlikely that King was aware of this communist connection and very unlikely that he was significantly influenced by any communist organization.
Source: Andrew, Christopher and Mitrokhin, Vasili "The Sword and The Shield: The Mitrokhin Archive and the Secret History of the KGB"
I think his potential involvement with communist groups was fabricated by J Edgar and the FBI. He never came out on the side of any political group. His problems with capitalism most likely have to do with how capitalism had been harmful to the African-American community