Booker and U.S. Rep. Tulsi Gabbard also did not participate in December’s Los Angeles debate.

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Five Democrats on stage is quite the reduction in number from the party’s first debates in 2019. Twenty Democrats took the stage over two nights, June 26-27, in Miami.

2019 November Democratic debate highlights

Ten Democrats were in Atlanta's November debate at Tyler Perry Studios.

Several of those Democrats in that first debate have dropped out in recent months: former U.S. Rep. Beto O'Rourke in October; U.S. Sen. Kamala Harris in December; and former Housing and Urban Development Secretary Julián Castro on Jan. 2.

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The January debate is being co-hosted by CNN and the Des Moines Register. This seventh Democratic debate will be held at Drake University.