Booker, Castro and Bloomberg likely to be blocked from January debate.

To make the stage, candidates must score 5 percent in four party-sanctioned national polls or 7 percent in two polls in early states. They must also prove that they have 225,000 individual donors, up from 200,000 in December, with more than 1,000 coming from 20 different states.

The donor requirement is prohibitive for Bloomberg, who has been polling near the cutoff but has vowed to accept no donations during his campaign. The polling levels could prove a challenge to businessman Andrew Yang and billionaire investor Tom Steyer, who made the December debate but have been polling under 5 percent in recent surveys.

Former vice president Joe Biden, Sen. Elizabeth Warren (D-Mass.), Sen. Bernie Sanders (I-Vt.), Sen. Amy Klobuchar (D-Minn.) and South Bend, Ind., Mayor Pete Buttigieg will make the cutoff.

The new rules are also likely to keep two nonwhite candidates off the stage for another round. Booker, who is black, has been polling under 4 percent and had asked the Democratic National Committee to consider other criteria, like early state endorsements and offices, so he could make the debate. Former Housing Secretary Julián Castro, who is Latino, is also unlikely to make the cut with those levels.

Democratic chairman Tom Perez had left open the possibility in interviews of changing the debate rules for January, which he has recently been deciding on a month-to-month basis. Last week, the top polling presidential candidates signed a letter to the party asking for looser qualifications to allow more candidates into the contest.

The first debates in the summer welcomed 20 candidates to the stage over two consecutive nights, but Perez has gradually been raising the polling and individual donor levels since then.

The seventh debate will be held at Drake University by the Des Moines Register and CNN, which was the first to report the new qualification levels. The Iowa caucuses will be held on Feb. 3.