Having these women by her side has provided Mrs. Clinton with powerful and deeply sympathetic character witnesses as she makes her case to African-American voters. And they have given her campaign, an often cautious and poll-tested operation, a raw, human and sometimes gut-wrenching feeling.

The presence of the mothers has also proved a shrewd political move, influencing black leaders and lawmakers to back Mrs. Clinton.

“Those not supporting her are reluctant to go against her, because we led the marches and the rallies on these things and have worked very closely with the mothers,” said the Rev. Al Sharpton, whose National Action Network is hosting Mrs. Clinton and her opponent in the Democratic primary, Senator Bernie Sanders of Vermont, for a discussion of civil rights issues at its annual conference this week in New York. “It certainly influences how we related to her campaign,” added Mr. Sharpton, who has not endorsed a candidate.

On Wednesday, Mrs. Clinton received the biggest applause of an otherwise lukewarm reception at Mr. Sharpton’s convention when she introduced Sybrina Fulton, the mother of Mr. Martin, and Gwen Carr, the mother of Mr. Garner.

Mr. Sanders has the support of Mr. Garner’s daughter, Erica; the director Spike Lee; Mr. West; and other prominent black figures, and he talks frequently about being arrested in the 1960s while marching for civil rights. But the mothers have allowed Mrs. Clinton to “really tap into the pulse of the black community,” said Representative Yvette D. Clarke, Democrat of New York, who has endorsed Mrs. Clinton.

“The other candidate on the Democratic side did not reach out to us,” Annette Nance-Holt, whose 16-year-old son, Blair Holt, was shot on a Chicago bus in 2007, said at a campaign event last month. She explained starkly that she was not swayed by Mr. Sanders’s promise of free college “because my child is dead.”