Here are the Democratic candidates left in the race, along with those who have dropped out along the way.
Michael Bennet, U.S. senator from Colorado
Entered race: May 2, 2019
Education: B.A., Wesleyan University; J.D., Yale Law School
Best known for: A scorching speech slamming Republican Sen. Ted Cruz of Texas during the recent government shutdown, then publicly announcing that he had prostate cancer in March.
Strength: Bennet is viewed as a wonky, issue-oriented pragmatist who has won tough campaigns, which could be a winning combination for Democratic voters who don't want to veer too far left but are eager to beat Trump.
Weakness: Bennet is not as well known as many other senators competing for the Democratic nomination and has no natural base.
Mike Gravel, former U.S. senator from Alaska (dropped out)
Entered race: March 19, 2019
Dropped out: July 31, 2019
Education: B.S., Columbia University
Best known for: Serving as U.S. senator representing Alaska from 1969 to 1981 who read the Pentagon Papers into the congressional record. He also ran for president unsuccessfully in 2008, first as a Democrat and then as a Libertarian.
Biggest strength: Long antiwar record, which could play well with the progressive wing of the party.
Biggest weakness: Oldest candidate in the race by far. Gravel's campaign has previously said he wanted to make the debate stage in an effort to push the party to the left, not to win.