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Cornel West, the influential scholar and civil rights activist, has endorsed Senator Bernie Sanders for president, bolstering a candidate who has drawn huge crowds but also skepticism from black voters.

Mr. West, a professor at Union Theological Seminary, explained in a series of Twitter messages on Monday night that Mr. Sanders, an independent from Vermont who is seeking the Democratic nomination, has been an ally in the fight for justice over the years and that his voice needs to be heard.

I endorse Brother @BernieSanders because he is a long-distance runner with integrity in the struggle for justice for over 50 years. — Cornel West (@CornelWest) August 25, 2015

The endorsement is a valuable one for Mr. Sanders, who has been gaining ground on Hillary Rodham Clinton in recent polls but who has faced protests from Black Lives Matter activists who have said he is not sufficiently supportive of their cause. This month, a group of activists cut short one of the Sanders rallies when they took the stage and commandeered his microphone.

Mr. West, who has been critical of President Obama’s efforts to uplift African-American causes, explained recently that Mr. Sanders would best serve black people because of his desire to clamp down on Wall Street and to invest in working people and poor people.

Despite his support for Mr. Sanders, who considers himself a socialist, Mr. West said that he was not affirming the “neo-liberal Democratic Party.” He also weighed in on Donald J. Trump’s candidacy, saying that while the billionaire developer possesses authenticity, he lacks integrity.