First of all is important try to contextualized the question.
The Congress passed civil rights acts during 1957,1960 and 1964, but none of these laws were strong enough to stop voting discrimination by local officials.
Around march of 1965 peaceful voting rights protester in Selma, Alabama were violently attacked by alabama state police. (Source Wikipedia).
In that act of civil rights laws more house democrats shifted from opposing to favoring the law.