Cory Booker has said the Democratic party’s process for determining who gets on the debate stage puts “elites” and “money” in control.
His remarks came as he fought to get the polling numbers to be on the debate stage in California on December 19.
The qualifying deadline is Thursday.
Mr Booker said the requirements that are for now keeping him off the stage do not reflect the enthusiasm he has seen from Iowa voters over a four-day tour across 12 counties.
A Democratic party spokeswoman said the qualifying thresholds have remained low and that the party’s process has been inclusive.
Meanwhile, Pete Buttigieg is facing intense scrutiny for the first time as his outsider Democratic presidential campaign continues to roll in early-voting Iowa and New Hampshire.
He has responded in his characteristic unflappable style, saying, “The more heat there is, I welcome that.”
Mr Buttigieg is being pressed, especially by rival Elizabeth Warren, to disclose corporate clients he served at a top consulting firm and open his fundraising events to the media.
Mr Buttigieg has asked McKinsey & Co to release him from a non-disclosure agreement and is weighing whether to disclose more about his key donors.