When the Democratic primary contest began last winter, it featured the most racially diverse field in history, with two black senators, a Latino former cabinet secretary, an Asian-American businessman and the first American Samoan elected to Congress . But 10 months later, the Democratic field has a top tier of four white candidates, three of them men.

Candidates of color are languishing in the low single digits in polls, well behind the four leaders — former Vice President Joseph R. Biden Jr., Senators Elizabeth Warren of Massachusetts and Bernie Sanders of Vermont, and Mayor Pete Buttigieg of South Bend, Ind. The divide could become even more pronounced in the coming months: Because of new, more rigorous thresholds for the Democratic debate in December, it is possible that only one nonwhite candidate, Senator Kamala Harris of California, will qualify.

With the Iowa caucuses three months away, it’s a noticeable sorting of the field for an increasingly multiracial party, whose most recent White House victories were powered by a strong minority turnout for a black nominee. It has frustrated the Democrats seeking a more diverse party leadership — not to mention the affected candidates.

“I’ve had lots of crazy things said to me, like, ‘Is America ready for another black president?’” Senator Cory Booker of New Jersey said in an interview. “And I’m confident it’s never been asked of a white candidate, ‘Is America ready for another white president?’”