 Joe Biden declared he "should not have been so cavalier" on Friday after he told a prominent black radio host that African-Americans who back President Donald Trump "ain't black".
The rebukes included allies of Mr Trump's re-election campaign and some activists who warned that Mr Biden must still court black voters, even if African-Americans overwhelmingly oppose the president.
"None of us can afford for the party or for this campaign to mess this election up, and comments like these are the kinds that frankly either make black voters feel like we're not really valued and people don't care if we show up or not," said Alicia Garza, a Black Lives Matter co-founder and principal of Black Futures Lab.
South Carolina Senator Tim Scott, a Trump supporter and the Senate's sole black Republican, said he was "shocked and surprised" by Mr Biden's remarks.
Sixty-one percent of black voters supported Mr Biden during the primary season, according to AP VoteCast surveys across 17 states that voted in February and March.
There is little chance of a sudden shift in support for Trump among black voters.
Mr Biden's selection of a running mate, along with his pledge to appoint the first black female Supreme Court justice, could help motivate voters?
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