There is some precedent for pundits to act as the referees in primary debates. Ms. Maddow co-moderated an MSNBC debate between Hillary Clinton and Senator Bernie Sanders of Vermont in 2016. Megyn Kelly, formerly of Fox News, whose prime-time show straddled news and commentary, co-moderated a Republican primary debate in August 2015.

Still, Ms. Maddow’s recent interviews of Democratic candidates have varied in tone.

In January, she grilled Senator Kirsten Gillibrand of New York on her previous support from the National Rifle Association, opening their conversation with a tough video laying out Ms. Gillibrand’s “unexpected turns” on guns and immigration.



Later that month, Senator Kamala Harris of California received a friendlier reception. Ms. Maddow concluded the interview by saying: “Senator Harris, I am excited to see the effect that you have on the primary process. I think that you are going to be a formidable contender. I will just say honestly, I think there is a good chance that you are going to win the nomination.”

Each installment of NBC’s Miami debate is two hours long. Only Mr. Holt will appear for both hours; Ms. Maddow, along with the other co-moderators, will join for a single hour each night.

The party has not yet announced which 20 candidates have qualified for the debate, a determination to be made by the Democratic National Committee based on a mix of national polls and quantity of campaign donors. Once chosen, the qualifying candidates will be divided into groups of 10 between the two nights.