Washington — The Reverend Jesse Jackson is backing Senator Bernie Sanders for the Democratic presidential nomination as he seeks to mount a comeback when voters in six states head to the polls Tuesday.

Jackson, a longtime civil rights leader and Democratic presidential candidate in 1984 and 1988, said in a statement he will appear alongside Sanders at an event in Grand Rapids, Michigan, on Sunday. He praised the Vermont senator for committing the tackling issues that are of significance to the African-American community.

"With the exception of Native Americans, African Americans are the people who are most behind socially and economically in the United States and our needs are not moderate. A people far behind cannot catch up choosing the most moderate path," Jackson said. "The most progressive social and economic path gives us the best chance to catch up and Senator Bernie Sanders represents the most progressive path. That's why I choose to endorse him today."

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Jackson said former Vice President Joe Biden's campaign did not reach out to him or ask for his support. Sanders, to the contrary, did. As mayor of Burlington, Vermont, Sanders backed Jackson in his 1988 campaign, in which Biden was also a candidate.

Jackson said Sanders made several commitments to him, including introducing a constitutional amendment on the right to vote in Congress next week, expanding Pell Grants, nominating an African-American woman to the Supreme Court and naming black women to his cabinet if he is elected president.

The civil rights leader also challenged Sanders to select an African-American woman as his running mate, which Jackson said the senator would give "the highest consideration."

Jackson's endorsement comes as the race for the Democratic presidential nomination has narrowed significantly, with Sanders and Biden emerging as the two frontrunners.

Biden surged in the national delegate race, which he now leads, following a sweep of Super Tuesday states, though California — the biggest prize — is leaning toward Sanders.

Sanders is now focusing on Michigan, a key battleground state where more than 100 delegates are up for grabs Tuesday.