Support for Joe Biden dropped 10 points after he struggled to defend his anti-busing record during the first Democratic presidential debate performance of his 2020 bid.

While the former vice president commanded 41.5% support among likely Democratic voters before the first presidential debate, spanning across Wednesday and Thursday, his support plummeted to 31.5% afterwards, according to a Morning Consult/FiveThirtyEight poll.

California Sen. Kamala Harris attacked Biden during Thursday's debate in Miami for working with segregationist senators in the 1970s to oppose federal desegregation busing.

"There was a little girl in California who was part of the second class to integrate her public schools, and she was bused to school every day. That little girl was me," Harris, who is of Jamaican and Indian descent, told Biden during the debate.

Biden argued that he has long supported and fought for civil rights and refused to apologize for his opposition to Education Department-mandated busing.

"I did not oppose busing in America," Biden said. "What I opposed is busing ordered by the Department of Education. That’s what I opposed."

In 1975, Biden argued that busing in principle was a "rejection of the whole movement of black pride."

Harris saw the largest gain in support in the poll, jumping nearly 9 points from 7.9% before the two debates to 16.6% after the debates.

Likely Democratic voters' opinions of Biden also dropped somewhat after his debate performance, with the poll showing 76.5% having a favorable opinion of Biden and 16.1% unfavorable pre-debate to 75.6% favorable and 19.1% unfavorable post-debate.

The poll determined candidates' pre-debate support from 7,150 respondents from June 19 to June 26, with a margin of error of 1 percentage point. The post-debate support came from 1,399 respondents who were part of the 7,150 pre-debate group on June 27 and 28, with a margin of error of 3 percentage points. Of those, 846 had watched the Thursday debate or both of the debates.