Financial support from mostly white, educated elites has helped propel the candidacies of Ms. Warren and Mr. Buttigieg, while two nonwhite candidates — Mr. Booker and Julián Castro, the former housing secretary — have had to explicitly petition supporters for money to stay in the race.

Assumptions about which candidates can or can’t beat President Trump have also hurt minority candidates, with some Democrats fearing that the white working class voters who helped propel Mr. Trump to the White House would not support a nonwhite nominee. Donors for Mr. Booker and Ms. Harris have argued that such implicit bias causes voters and pundits to treat Mr. Buttigieg as the natural heir to Mr. Biden’s moderate coalition, though their candidates perform better among older black voters, the Democrats’ most loyal moderate voting block.

Other Democrats, however, say the blame rests with the candidates themselves. None of the nonwhite challengers have been able to forge a strong coalition or peel away black voters from Mr. Biden. Some voters and elected officials, white and nonwhite alike, now say privately what the polling suggests: They have not been swept off their feet by any of the candidates of color.