So far, 19 candidates have qualified to be in the first debate next month, creating a situation where most of them are unlikely to get much airtime. For that debate, the committee agreed to evenly and randomly divide top-tier candidates across two nights in order to maintain viewer interest.

The third debate will be shown by ABC News, in partnership with Univision, and will be held on two consecutive nights — Sept. 12 and 13. It will be capped at 20 candidates, but it is unlikely that many will qualify. Currently, only about eight candidates consistently reach 2 percent in the polls.

Party officials say the additional two months of campaigning, combined with appearances in the heavily promoted summer debates, could lead to more candidates reaching the new threshold. Officials also point out that no Democratic candidate consistently polling under 2 percent has gone on to win primaries and caucuses since President Jimmy Carter in 1975.

Senator Cory Booker of New Jersey quickly used the new rules to solicit donations, telling supporters on Wednesday that his campaign has “some work to do to hit this number.” Senator Kirsten Gillibrand’s campaign also sent a fund-raising blast to supporters, citing the donor requirement and saying, “This is going to be tough — but we can do this.’’

Senator Kamala Harris of California, before an event in Greenville, S.C., also expressed some skepticism. “I’m not in a position to tell the D.N.C. what to do, but I think that there is no question that we need to support a robust process of letting everyone make their case to the American people,” she said. “At some point it will whittle down, and that’s part of what a competition involves.”