The radicalism of Dr. Martin Luther King had to do with his economic politics and philosophy. He would be what would be described as a Social Democrat both as it related to civil and equal rights. But when it came economic policy and having a big central government there to reduce poverty and what is called income inequality and that type of economic philosophy is very radical in America. Even though it is very mainstream in Scandinavia which is full of social democracies.
The 1960s at least the early and mid 1960s was about equal rights under law the civil rights movement. But by the late 1960s after the passage of those very important laws, it became about the income gap and the lack of educational and economic opportunities not just for African-Americans, but other Americans as well living in poverty regardless of race. Which was what the Poor People’s Campaign was about that would’ve been the second act of Dr. King’s movement.