Short of a dramatic turnaround in his fortunes, the 5 percent threshold will doom Mr. Booker’s chances of returning to the debate stage. He has not reached 5 percent in a qualifying poll since March. The former housing secretary Julián Castro, the only Latino candidate in the race, has never reached 5 percent.

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Patrick Murray, director of the Monmouth University Polling Institute, which puts out one of the surveys that can help candidates qualify for debates, said it currently only plans to release one qualifying poll before the January deadline. He added that it seemed unlikely that a total of more than four or five qualifying polls will be released in time for the debate next month.

“We saw a big gap in polling output because of Thanksgiving for this last debate,” he said. “Now we’ve got an even longer holiday period where I don’t think a lot of people are going to poll.”

The January debate, scheduled to take place at Drake University, could be moved or postponed if President Trump’s impeachment trial in the Senate is taking place in mid-January. It remains unknown when the trial will begin or how long it will last.

Mr. Murray noted that the focus on impeachment could tilt pollsters toward conducting national surveys rather than ones in early voting states, which could hurt candidates who have had no choice but to invest heavily in advertising in places like Iowa and New Hampshire.

Three more debates are planned for February — one each in New Hampshire, Nevada and South Carolina.

Mr. Booker on Saturday organized eight other Democratic presidential candidates to sign a letter to Tom Perez, the Democratic National Committee chairman, asking that debate thresholds be lowered in a way that would allow him and Mr. Castro to participate in the January and February debates.