The D.N.C. did not say Thursday how it would determine which candidates would qualify for the next set of debates. It has ratcheted up its polling and fund-raising standards for the six debates this year, beginning with a low threshold that resulted in 20 candidates qualifying for the first two debates and gradually raising the bar.

The debates announced on Thursday will be divided among familiar television networks, including ABC, CBS, CNN and NBC. But for the first time in the 2020 race, a pair of major technology companies signed on as sponsors, too. Apple News will co-host a debate in Manchester, N.H., on Feb. 7 with ABC News, and Twitter will partner with CBS News and the Congressional Black Caucus for a Feb. 25 event in Charleston, S.C.

Sign up for On Politics to get the latest election and politics news and insights. Sign up for our politics newsletter

Absent from the sponsorship list was Facebook, whose role in allowing disinformation to proliferate in the 2016 race has been criticized by some leading Democrats.

The inclusion of Apple and Twitter could create an awkward dynamic for candidates who have made stricter regulation of the tech industry a component of their campaign platforms. Senator Elizabeth Warren of Massachusetts, for instance, has called for Apple to be broken apart. And some candidates have faulted Twitter for what they see as lax policies toward political speech and unfounded rumors.