Records show Bernie was an active member of Chicago’s Congress of Racial Equality (CORE) and Chairman of the Social Action Committee, and also an organizer of the Student Non-Violent Coordinating Committee.
I believe both organization appear on the FBI and Congress’s list of subversive organizations. I’m not sure that is the correct name of the list - but these are people who pursued the downfall of the US government, violently if necessary. According to what I knew at the time, they were supported by Russia with dollars which came from Russia by way of Cuba and Canada. Some of the leaders were known to liaison with international Communists. Whether Bernie was involved in that part would be a good question to ask.
CORE is one of the GOOD civil rights organizations, last I heard. Am I wrong?