(Dan Anderson/EPA)

The Rev. Jesse Jackson Sr.  endorsed Cook County Commissioner Jesus "Chuy" Garcia on Monday for mayor of Chicago, throwing his support to the underdog looking to unseat incumbent Rahm Emanuel in a race that is shaping up as the latest front in the battle over the future of the Democratic Party.

Jackson's decision puts him at odds with President Obama, who backs Emanuel, his first White House chief of staff.

The endorsement comes less than a month until the April 7 election. Jackson, a prominent African American civil rights activist, could boost support for Garcia in the all-important battle for black voters. Garcia also won the backing of Rep. Danny Davis (D), a black congressman, on Sunday.

“Chuy was one of my delegates when I ran [for president] in '88,” Jackson recently told the Chicago Sun Times, adding, "I'm very inclined toward Chuy because of our relationship and what he represents."

Emanuel did very well among black voters in 2011, but support for him slipped in majority African American wards in last month's election. Emanuel failed to win a majority of the vote in the Feb. 24 election, forcing him into a runoff against second-place finisher Garcia.

Once a little-known politician, Garcia has waged a surprisingly competitive populist campaign against the mayor, hitting him for closing dozens of schools, criticizing his record on crime and accusing him of prioritizing the interests of the city's wealthy residents over everyone else.

Emanuel has countered by pointing to his first-term achievements, including a minimum wage increase and expanding full-day kindergarten. He has also turned to the nation's first black president to help him make his case for a second term.

Obama visited Chicago just days before the Feb. 24 election to dedicate the city's first national monument in an area where African American porters organized the nation's first black labor union. He also swung by an Emanuel campaign office and has appeared in ads for the mayor.