Joseph R. Biden Jr. entered the 2020 presidential campaign with wide support from black voters and a reservoir of good will stemming from his eight years as an unswervingly loyal vice president to Barack Obama — a huge head start in a primary where African-Americans will play a decisive role.

But as the next high-stakes televised debate rapidly approaches, the two leading black candidates in the Democratic contest, Senators Cory Booker and Kamala Harris, have mounted a direct challenge to Mr. Biden’s decades-long record on race in hopes of undermining his standing with black voters and ultimately derailing his candidacy.

The reckoning over Mr. Biden’s record has drawn him into the most direct confrontation of the campaign yet, as Ms. Harris and Mr. Booker try to make the case to black voters that they are better suited to promote their interests than Mr. Biden . The two candidates have criticized Mr. Biden for reframing his own history, including his previous opposition to busing and his role in drafting the 1994 crime bill .

Early polling, both nationally and in some key early states, indicates that many black voters remain unmoved. On Thursday, a Monmouth University poll from South Carolina, a key early voting state, found that 51 percent of black voters there supported Mr. Biden; 12 percent supported Ms. Harris, and just 2 percent supported Mr. Booker.