Footage has emerged showing a young Bernie Sanders being arrested during a civil rights protest in Chicago in 1963.
The demonstration was against the University of Chicago having segregated housing, a form of discrimination against African-American students.
Tad Devine, a senior advisor to the Sanders campaign, told the Tribune that "Bernie identified it himself”.
"He looked at it — he actually has his student ID from the University of Chicago in his wallet — and he said, 'Yes, that indeed is (me).'"
Sanders’ involvement in the civil rights movement has been a point of discussion during his run for the White House.
Lewis Smith, one of the most visible leaders during the civil rights movement and now a Congressman in Georgia, appeared to question the Senator last week.
Mr Smith later clarified his statement: “The fact that I did not meet him in the movement does not mean I doubted that Senator Sanders participated in the civil rights movement. Neither was I attempting to disparage his activism.”
Focus now turns to South Carolina, the next location for the Democratic primary, which takes place on Saturday.
There is a strong African-American voting base in the state which both Sanders and Clinton have prioritised in the lead-up to the primary.
The endorsement came in the form of a powerful four-minute campaign advertisement, which can be viewed above.
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