Democratic presidential hopeful Bernie Sanders on Thursday released a four-minute advertisement featuring the oldest daughter of Eric Garner, whose death in New York from a police chokehold helped inspire the Black Lives Matter movement.

The ad tells the story of Erica Garner's life and activism in the wake of her father's death, and it concludes with her explaining her support for Sanders's presidential bid: "I'm behind any who's going to listen and speak up for us, and I think we need to believe in a leader like Bernie Sanders," she says.

The Vermont senator's campaign debuted the ad on the same day that several leading African American members of Congress supporting Hillary Clinton questioned the depth of his commitment to the civil rights movement, amid a stepped-up emphasis on courting black voter by both Democratic candidates.

After nominating contests in two largely white states -- Iowa and New Hampshire -- African Americans are a key constituency in upcoming contests, particularly in the South.

In a conversation with reporters before today's Democratic debate, Sanders strategist Tad Devine said he had read Garner's endorsement of Sanders in a Washington Post column and was "blown away," quickly asking his partners if they could interview her. They trekked to New York and recorded the spot.

Michael Briggs, a Sanders spokesman, said his campaign had sought to buy four-minute slots to air the entire Garner ad on several television stations, including CNN, MSNBC and BET, but it was told the stations would not sell that much time.

"We are now trying to edit it and get on the air ASAP," Briggs said in an email.

Sanders, who represents a state that is 95 percent white, has acknowledged from the outset of the campaign he faces a challenge in connecting with minority voters. He and his team have unveiled some notable endorsements in recent days, including Benjamin Jealous, former head of the NAACP, and Harry Belafonte, an entertainer and longtime civil rights activist.

Sanders, who was heckled by Black Lives Matter protesters over the summer, now regularly includes a section in his stump speech about combating institutional racism and addressing police brutality. Clips from his speeches and a debate appearance are included in the new ad.

Sanders also often notes his history of civil rights activism dating back to his college days in the 1960s, including his attendance at the March on Washington led by Martin Luther King Jr. in 1963, and his arrest for protesting segregated housing at the University of Chicago.

At a news conference Thursday, several members of the Congressional Black Caucus questioned Sanders's ties to the black community as their political action committee formally endorsed Clinton. Rep. John Lewis (D-Ga.), a civil rights icon, said he never encountered Sanders during the 1960s-era movement against segregation.

“I never saw him. I never met him,” Lewis said at a news conference.

In the new Sanders ad, Erica Garner asserts that Sanders has been more committed than any other presidential candidate on the issues facing her community these days.

"There's no other person that's speaking about this," she said. "We need a president that's going to talk about it."

On Thursday, Devine said that the Sanders campaign had already recorded testimonial spots from "elected officials and leaders who've endorsed Bernie Sanders," and are prepping them to air in South Carolina. They would include some of the African-American state representatives who'd endorsed Sanders in recent weeks, each one of them a surprise to the allies of Hillary Clinton.

Meanwhile, in the run-up to tonight's debate, Clinton's campaign announced the endorsement of Geneva Reed-Veal, the mother of Houston woman Sandra Bland, who was discovered hanged in police custody after having been pulled over in a traffic stop.