Only 10 Democratic candidates are expected to qualify for the debate in September, significantly shrinking the debate stage from the summer and giving them the first chance to all appear on the same night.

Two more qualifying polls from Quinnipiac University and USA Today/Suffolk University were released Wednesday – the deadline to qualify for September's debate – but billionaire donor and environmental activist Tom Steyer, who needed to reach a threshold of 2% in one more poll to qualify, didn't. He garnered under 1% in USA Today's poll and got 0% in Quinnipiac's survey.

The massive Democratic field of 21 candidates is unlikely to see an immediate wave of drop-outs, despite the debate stage being cut in half since the June and July debates. Candidates who missed out still have weeks left to qualify for October's debate, and a few are close to returning later in the fall.

To qualify for both the September and October debates, candidates must reach at least 2% in four approved national or state polls and receive contributions from 130,000 unique donors. All 10 candidates who have already qualified for September are guaranteed a spot in October.

The upcoming debate on Sept. 12 in Houston will feature former Vice President Joe Biden, Sen. Bernie Sanders of Vermont, Sen. Elizabeth Warren of Massachusetts, Sen. Kamala Harris of California, Sen. Cory Booker of New Jersey, former Rep. Beto O'Rourke of Texas, Sen. Amy Klobuchar, former Housing and Urban Development Secretary Julian Castro, tech entrepreneur Andrew Yang and South Bend, Indiana, Mayor Pete Buttigieg.

The debate marks the first time that all the front-running candidates will appear on the same stage, and it will be the first match-up between Biden and Warren, who has been surging in the polls over the past few weeks. That'll likely expose the large ideological rifts between some of the top-tier candidates, especially on the issue of "Medicare for All."

Candidates who've long known they'd be left off of the debate stage have fumed over the tighter qualification requirements, arguing that the donor threshold allows wealthy challengers to unfairly use their money. Montana Gov. Steve Bullock has lashed out at Steyer, who amassed his wealth as a hedge fund investor, and said that the Democratic National Committee's "well-intentioned" rules have ultimately allowed a billionaire to "buy a spot" on the stage.

The New York Times reported that Steyer has spent $12 million on ads running online, mainly on Facebook, and on TV over the course of six weeks.

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For those hopeful for a debate stage return, the next deadline is two weeks before October's debate, though the date and location have yet to be announced. Other than Steyer, who needs one more poll, a few other Democratic challengers have a shot at a debate comeback.

Rep. Tulsi Gabbard of Hawaii is two polls away from qualifying and has already hit the donor requirement. Author Marianne Williamson has also hit the donor threshold but needs to reach the polling threshold in three more surveys to qualify. Sen. Kirsten Gillibrand of New York needs three more polls to qualify and still needs to hit the donor requirement.

For the rest of the field, it's unlikely many will appear in a future primary debate. Three candidates who weren't going to make September's debate recently dropped out of the race and decided to pursue running for other races: former Colorado Gov. John Hickenlooper, Washington Gov. Jay Inslee and Rep. Seth Moulton of Massachusetts.