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.