WASHINGTON (Reuters) - When 20 Democrats take the stage next week for the first of 12 prime-time presidential primary debates, they will assemble according to months-in-the-making plans by the Democratic National Committee to accommodate its largest-ever field.

FILE PHOTO: Twenty 2020 Democratic presidential candidates who will participate in the party's first debate in Miami later this month in a combination file photos (L-R top row): U.S. Senators Bernie Sanders, Kirsten Gillibrand, Cory Booker, Kamala Harris, Amy Klobuchar, Michael Bennet, and Elizabeth Warren. (L-R middle row): Former U.S. Vice President Joe Biden, Mayor Pete Buttigieg, New York City Mayor Bill de Blasio, former HUD Secretary Julian Castro, Former Gov. John Hickenlooper, and Gov. Jay Inslee. (L-R bottom row): Entrepreneur Andrew Yang, U.S. Representatives Tim Ryan, Tulsi Gabbard, Eric Swalwell, Author Marianne Williamson, Former Representatives John Delaney and Beto O'Rourke. REUTERS/Files/File Photo

For two hours each on the nights of June 26 and 27, the candidates seeking the party’s 2020 nomination will go head-to-head at a performing arts center in downtown Miami in debates broadcast live by NBC, MSNBC and Telemundo.

It will be an opportunity for some of the candidates to get their biggest national exposure yet - and for voters to start making decisions about which White House hopeful they will back.

Past studies have shown that as many as 80% of voters have not settled on a candidate going into a first primary debate, mainly because they did not know many of the candidates yet, said Mitchell McKinney, a political communication professor at the University of Missouri who studies presidential debates.

“That starts to firm up, and after a series of primary debates we start to see people pointing to the candidates that they’re likely to support in the primary,” McKinney said.

Here is what you should know as the Democratic candidates prepare to debate for the first time:

The party chose the format with fairness in mind.

When there was a large field of Republican candidates vying for the 2016 nomination, the Republican National Committee opted to hold back-to-back debates on the same day: an earlier “undercard” debate of lesser-known candidates and a “main stage” debate in prime-time of the front runners. The earlier debate had only a fraction of the viewership as the prime-time debate.

DNC spokeswoman Xochitl Hinojosa said party Chairman Tom Perez met with potential media partners, former party officials, former presidential candidates, consultants and other stakeholders over the past 18 months to determine what would be fairest to the large field of Democratic candidates.

That’s how the party settled on holding debates over two consecutive nights and picking the line-up for each by random drawings.

“We wanted to make sure we were putting all the candidates on a level playing field,” Hinojosa said.

Candidates had to reach polling and donor thresholds to qualify.

To make it onto the debate stage, candidates had to meet criteria set by the DNC, which required either reaching 1% support in three qualifying national polls; or receiving donations from at least 65,000 unique individuals, with 200 donors in at least 20 states.

The candidates were assigned stages by a drawing.

NBC last week held a random drawing in New York City. Officials had two boxes, one containing the names of candidates who polled at 2% or above and the other those who polled below the 2% threshold.

The names from the first box were selected at random and distributed between the two nights, and then the same process was followed for the second box to ensure a mix of candidates on each night, those familiar with the process said.

On each night, leading candidates will be center stage.

The candidates’ podium positions on stage will reflect their standings in polls as of June 12, with leading candidates in the center positions and trailing candidates at the edges, NBC announced this week.

U.S. Senator Elizabeth Warren of Massachusetts and former Representative Beto O’Rourke of Texas will be center stage on the first night, debating Senator Cory Booker of New Jersey, former U.S. housing secretary Julián Castro, former Representative John Delaney of Maryland, New York City Mayor Bill de Blasio, Representative Tulsi Gabbard of Hawaii, Washington Governor Jay Inslee, Senator Amy Klobuchar of Minnesota and Representative Tim Ryan of Ohio.

On the second night, former Vice President Joe Biden and Senator Bernie Sanders of Vermont, the race’s front-runners, will be in the middle. Joining them on stage will be Senator Michael Bennet of Colorado; Mayor Pete Buttigieg from South Bend, Indiana; Senator Kirsten Gillibrand of New York; Senator Kamala Harris of California; former Colorado Governor John Hickenlooper; Representative Eric Swalwell of California; author Marianne Williamson and entrepreneur Andrew Yang.

Four candidates did not make the cut.

Montana Governor Steve Bullock, former Senator Mike Gravel of Alaska, Representative Seth Moulton of Massachusetts and Wayne Messam, mayor of Miramar, Florida, did not meet the DNC’s qualifying criteria.

Bullock, who has said his current elected role prevented him from entering the presidential race earlier, has gone on a media blitz saying he was “ousted” from the debate stage and is planning his own town halls.

(The story corrects threshold in drawing in paragraph 12.)