The latest Morning Consult weekly tracking poll, conducted July 1-7, found Biden’s first-choice support among black Democratic primary voters has dropped 8 percentage points, to 38 percent, since a poll conducted in the week before the first primary debates. Over the same period, California Sen. Kamala Harris increased her first-choice support with the key Democratic demographic by 7 points, to 16 percent, while Vermont Sen. Bernie Sanders (I) gained 4 points, to 21 percent.

The movement came as Biden’s campaign reeled from the punch Harris packed against him during the June 27 Democratic debate. On stage, Harris criticized Biden’s decades-old opposition to federal involvement in busing as well as comments he made at a June fundraiser about working civilly with segregationists while he was in the Senate.

Biden, who was first elected to the Senate from Delaware in 1972 and was re-elected five more times, accused Harris of a “mischaracterization of my position across the board” during the debate, but it wasn’t until this past weekend — nine days later — that he expressed regret for his comments about the Senate segregationists.

“I regret it. I’m sorry for any of the pain or misconception they may have caused anybody,” he said in remarks Saturday in South Carolina to applause, a day before the latest survey concluded.

In the same speech, he sought to downplay his record from his early years in the Senate and instead highlight his record serving as vice president to former President Barack Obama, the nation’s first black president who remains popular with Democratic voters.

The controversy has not changed Biden’s favorability among black voters, with 76 percent expressing a favorable opinion of him and 12 percent expressing an unfavorable view in the latest poll.