CNN, which is broadcasting the debates, assigned candidates randomly with a drawing Thursday night, with 20 candidates spread evenly over two nights, July 30-31.
Former Colorado Gov. John Hickenlooper will appear the first night and U.S. Sen. Michael Bennet on Night Two.
Hickenlooper will join three top-tier candidates on July 30: U.S. Sen. Elizabeth Warren of Massachusetts, Sen. Bernie Sanders of Vermont and South Bend, Indiana, Mayor Pete Buttigeg.
Sanders, a self-avowed Democratic socialist, has come under fire from Hickenlooper, who has cautioned his rival candidates against embracing socialism.
And on July 31, Bennet will share screen time with U.S. Sen. Kamala Harris of California, former Vice President Joe Biden and Sen. Cory Booker of New Jersey.
This time, Harris, the lone black woman in the field, will be joined by another top black candidate, Booker, who also has been an outspoken critic of Biden.
Booker had denounced Biden for his recollections of the "civility" of working in a Senate that included white supremacists and for his leadership on a 1994 crime bill that the New Jersey senator assailed as a mass incarceration agent in the black community.
Meanwhile, Sanders and Warren share the July 30 lineup, allowing the two progressive icons to compete directly for the affections of the party's left flank.
They will be joined by several more moderate candidates, like Hickenlooper, who are likely to question the senators' sweeping proposals for single-payer health insurance and tuition-free college, among other plans.
Biden vs. Harris has quickly become the defining candidate-on-candidate juxtaposition in the early months of the contest.
Although of different sexes, races and generations, the two rivals share the same broad path to the nomination, particularly the broad coalition of white and black voters necessary to win the Southern primaries that dominate the early months of the nominating calendar.
Harris' June attacks on Biden's 1970s opposition to federal busing orders as a way to desegregate public schools was a way for her to stand out to liberal whites and to try to cut into Biden's strength in the black community, where he is lauded as the loyal vice president to Barack Obama, the nation's first black president.
To be clear, Biden aides say Harris' broadsides sparked a new aggressiveness and determination for the former vice president, and he's gone on a policy offensive in recent weeks, most notably on health care.
A proponent of adding a public option to the Affordable Care Act insurance exchanges, Biden almost certainly will try to pin down Harris on her support for Sanders' "Medicare for All" proposal. Harris, though, has stopped short of Sanders' explicit call for abolishing private insurance, and she insists that the plan can be paid for without any tax hikes on the middle class.
Also debating on July 30: Montana Gov. Steve Bullock (who did not qualify for the Miami debate); former Maryland Rep. John Delaney; Minnesota Sen. Amy Klobuchar; former Texas Rep. Beto O'Rourke; Ohio Rep. Tim Ryan; and author Marianne Williamson.
The July 31 lineup also includes New York City Mayor Bill de Blasio; former Obama Cabinet member Julián Castro; New York Sen. Kirsten Gillibrand; Hawaii Rep. Tulsi Gabbard; Washington Gov. Jay Inslee; and entrepreneur Andrew Yang.
Hickenlooper and Delaney have been among the most outspoken moderates warning Democrats against a leftward lurch. Klobuchar, Bullock and Buttigieg also position themselves as more centrist than Warren and Sanders.
A generational split also will be on display: Buttigieg, 37, and O'Rourke, 46, each have called for the party to pass the torch, while Sanders, at 77, is more than twice the young mayor's age. Warren, meanwhile, recently turned 70.
It will be the first debate opportunity for Bullock, who takes the spot that California Rep. Eric Swalwell had in June before dropping out in recent weeks. Another late entry to the race, billionaire activist Tom Steyer, did not meet the polling or fundraising thresholds required for the July debate.
For several of the longshot candidates, including the two Coloradans, the July debates are critical. The Democratic National Committee is doubling the polling and fundraising requirements to make the stage in the next round of debates, scheduled for September in Houston and October in a city yet to be announced.
As of now, it's likely those higher standards would mean many of the 20 candidates on stage in Detroit won't have a place in Houston.