Democrats Have 20 Spots in the First Debate. There Are Now 21 Candidates.

During the 2016 presidential primary, leaders of the Democratic National Committee were frequently accused of showing favoritism toward Hillary Clinton over Senator Bernie Sanders of Vermont. Perhaps as a result, party officials have tried to make the 2020 primary process appear more inclusive and transparent.

The first Democratic presidential debate showcases that effort: Candidates can gain access to the stage through grass-roots fund-raising, and in anticipation of a “historically large primary field,” officials decided to split the event across two nights in late June, so that as many as 20 candidates could take part.

But as of Thursday morning, when Senator Michael Bennet of Colorado announced his candidacy for president, those efforts at inclusion have been placed under strain. Mr. Bennet’s entrance ticks the number of Democratic candidates in the field up to 21, and additional candidates may still join, making it a real possibility that some will be left off the debate stage.

Only 17 candidates have so far qualified for the first debate, so cuts are not guaranteed. But with nearly two months to go, more candidates could very well meet the requirements.

Adrienne Watson, a spokeswoman for the D.N.C., said that the party has no plans to raise the cap from 20 or revise its debate qualification criteria.

“The D.N.C., along with its network partners, released the threshold for the first two debates nearly four months prior to the debates,” Ms. Watson said. “It will not be revised now.”

So what happens if more than 20 candidates meet the threshold? We’ll walk you through it.

An analysis by The Times earlier this week explained the two routes to qualification:

• A candidate receives donations from 65,000 people, including 200 donors apiece in 20 states.

• A candidate registers 1 percent support in three polls. (Only polls from a preset list of organizations are accepted.)

Based on these criteria, The Times found that 17 candidates had qualified for the debate stage.

The four who have yet to qualify are: Mr. Bennet; Representative Seth Moulton of Massachusetts; Mayor Wayne Messam of Miramar, Fla.; and the self-help author Marianne Williamson.

Along with Mr. Bennet, Mr. Moulton recently joined the race, and both have more than a month left to build enough momentum to qualify.

Ms. Williamson’s campaign said Wednesday that she needed just 6,300 more contributions from unique donors to get in, and Mr. Messam would need to register 1 percent — a low bar — in two more polls.

In other words, it is not inconceivable that more than 20 candidates could pass at least one of the thresholds.

If more than 20 candidates do manage to qualify, the D.N.C. has said it will decide who gets left out using what are essentially three tiebreakers.

In order of primacy, they are:

• Meeting both the donor and polling thresholds

• Highest polling average

• Most unique donors

The Times’s analysis found that nine of the 17 qualified candidates have already met both thresholds, and would qualify first.

In what is perhaps a signal of how badly several of the other candidates want to reach that safe harbor, some who are still short of the 65,000-donor mark have reached out with pleas for help.

“I need 65,000 donors to participate in the debates, and I am so close I can almost touch that stage,” the former housing secretary Julián Castro wrote in an email to supporters on Monday.

The same day, Senator Kirsten Gillibrand’s team sent an email warning. “With 20 declared candidates in this race, the D.N.C. may limit the list of presidential debate participants based on who does and doesn’t have 65,000 individual donors to their campaign,” they wrote. “Kirsten needs your help!”

Exactly how the D.N.C. will administer these tiebreakers is not clear. Its statement on the debates contains just one sentence about how participants will be selected if more than 20 candidates qualify, and it does not offer specifics on matters like how polling averages will be calculated or when the committee will stop accepting new data.

The Times’s analysis found that eight candidates had qualified only through their performance in polls, and had not reached the donor threshold.

Of those eight, Senator Cory Booker of New Jersey, who has received an average of about 2.6 percent support in the national polls in the analysis, seems relatively safe from getting cut.

Others, like Gov. Jay Inslee of Washington and Representatives Tim Ryan and Eric Swalwell, each average less than half of 1 percent — making them appear more at risk.

However, it is not clear which qualifying polls the D.N.C. will use to come up with an average, and a lot could change in the eight weeks between now and the first debate.