Candidates can hit that polling mark with either four different polls (nationally or from early states) of at least 5 percent, or two polls from Iowa, New Hampshire, South Carolina or Nevada of at least 7 percent. All polls have to be from the DNC's approved list of sponsor organizations, and either from separate sponsors or from different regions if relying on two polls from the same sponsor.

Biden, Buttigieg, Klobuchar, Sanders and Warren met those thresholds with weeks to spare, but Steyer hit the polling threshold just one day before the DNC's deadline to qualify.

While businessman Andrew Yang and New Jersey Sen. Cory Booker both hit the donor threshold, they couldn't hit the requisite number of polls and will not be on the stage.

Yang appeared on the December debate stage, while Booker last appeared on the stage in November. Booker dropped out of the race on Monday.

Former New York City Mayor Michael Bloomberg has hit the polling threshold, but the billionaire is not soliciting individual donations, which will make it all but impossible for him to qualify.

The Des Moines debate starts at 9 p.m. ET and is sponsored by CNN and the Des Moines Register.