Answer: "Voter discrimination."
Context:
In September, 1963, a bombing at a church in Birmingham sparked the African American community into strong action in Alabama to push for the rights of black citizens. As noted in a Voting Rights Timeline provided by Alabama State University, "the murder of four black girls at the Sixteenth Street Baptist Church in Birmingham" was the catalyst that launched a "new thrust on voting rights by the Southern Christian Leadership Conference (SCLC)."
The voting rights campaign in Alabama culminated in the Selma to Montgomery March in March, 1965. Martin Luther King, Jr., as president of the SCLC, was a key participant in that campaign, as was John Lewis, chairman of the Student Nonviolent Coordinating Committee (SNCC). Marchers (including Lewis) were attacked and beaten by state troopers the first time they attempted the march. Many Americans were sickened by the scenes of this which they saw on television, and National Guard troops then were deployed to protect the marchers as they again made their effort. There were roughly 2,000 (not 20,000) who made the successful march from Selma to Montgomery, March 21-25, 1965. When they arrived in Montgomery, they were met by a crowd of nearly 50,000 supporters, both blacks and whites in that supportive group.
