WASHINGTON — Cory Booker, in danger of being excluded from the Democratic presidential debate for the second straight month, blasted party officials for setting qualifications that he said were relegating candidates of color to the sidelines.

“Once again, we’re facing the prospect of a debate stage that is completely missing the perspectives of diverse communities,” Booker said in an email to supporters.

“We shouldn’t have debate rules that, on the one hand, allow self-funding billionaires like Tom Steyer to buy their way onto the stage, while, on the other, excluding qualified candidates of color from the conversation.”

Booker has yet to hit the polling threshold to get on the stage for the seventh debate, to be held Jan. 14 in Des Moines, Iowa.

Neither Deval Patrick or Julian Castro have qualified either, raising the real possibility that the January field, like the December one, will be almost all white. Entrepreneur Andrew Yang, who is Asian American, made the December debate but has yet to meet the polling threshold for January.

After seeing most of the minority candidates left off the stage in December despite beginning the 2020 campaign with the most diverse field of presidential candidates in history, Booker and other candidates asked the Democratic National Committee to adopt rules for January that would allow more people to qualify.

Instead, the DNC tightened them further.

“Too much is at stake in this election for the challenges facing communities of color not to be given the attention they deserve,” Booker wrote. “We are the party of diversity. We are the party of inclusion. Let’s start acting like it.”

DNC officials did not immediately respond to a request for comment.

Booker has met the requirement that he receive donations from 225,000 unique individuals, but has yet to reach 5 percent in any of the four polls (or 7 percent in two early-state surveys) needed to qualify.

His failure to meet the polling threshold also led to his exclusion from the December debate. Booker is at 2.5 percent in the Real Clear Politics polling average, putting him in eighth place.

Oddsmakers said it was unlikely Booker would get the support he needed to make the January stage. He was at 25 to 1 to qualify, according to US-Bookies.com, which owns https://bet-nj.com and aggregates data from major bookmakers.

Yang was at 3 to 1, Steyer at 5 to 1, Michael Bloomberg at 20 to 1 and Tulsi Gabbard at 33 to 1.

To try to raise his poll numbers in Iowa, where he is concentrating his efforts, Booker began a six-figure television advertising campaign this week.

The latest odds by US-Bookies.com made Booker a long shot to be the Democratic nominee, putting them at 150 to 1. Joe Biden was the favorite at 2 to 1.

“After already failing to qualify for the December debate, these odds suggest Booker should drop out of the race,” US-Bookies betting analyst Alex Donohue said.

“His odds to win the nominee and the election have been lengthening since November, meaning he continues to drift farther away from the pack.”

Jonathan D. Salant may be reached at jsalant@njadvancemedia.com. Follow him on Twitter @JDSalant or on Facebook. Find NJ.com Politics on Facebook.

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