Ta-Nehisi Coates has a piece called "Civil-Rights Protests Have Never Been Popular," in which he notes that in 1966, 63% of people polled had a negative opinion of Martin Luther King. The question was asked five times by the Gallup Poll, asking people to rate him on a scale of +5 to -5. A summary of the results, plus some historical events:
+ - -5
May 1963 41% 37% (20%)
March on Washington 8/1963
Aug 1964 44% 38% (22%)
Selma march, 3/1965
May 1965 45% 46% (27%)
Chicago open housing movement, mid- 1966
Aug 1966 33% 63% (39%)
Aug 2011 95% 4% (1%)
During King's life, there was always a significant number giving him the lowest possible rating, which I show in parentheses.
There are some complications, which I will discuss in the future, but Coates is right in his general point--King was not particularly popular when he was alive, and among whites negative views probably always outnumbered positive views. Strongly negative views were definitely more common than strongly positive views.
[Data from the Roper Center for Public Opinion Research]