The debate will feature at least five candidates — former Secretary of State Hillary Clinton, Vermont Sen. Bernie Sanders, former Maryland Gov. Martin O'Malley, former Virginia Sen. Jim Webb, and former Rhode Island Gov. Lincoln Chafee.
Another Democratic candidate, campaign finance reform activist and Harvard law professor Lawrence Lessig, will not be allowed to participate. This is because he didn't hit 1 percent or above in three national polls from CNN's approved list of organizations conducted between August 1 and October 10. (Indeed, he wasn't even offered as an option in most of these polls, which seems somewhat unfair — though when he is listed, he usually polls at 0 percent.)
It is unlikely, but technically possible, that Vice President Joe Biden could participate too. CNN deliberately designed its debate rules to make it as easy as possible for him to make the stage, even if he decided on a run very late. Since he's already met the polling requirements, all he has to do is say he'll declare his candidacy to the FEC by Wednesday, October 14 — the day after the debate.
So theoretically, if Biden is sitting around on the afternoon of debate day, he could make a spur-of-the-moment decision that he wants to jump in the race, jet off to the event in Las Vegas, and say he'll handle his paperwork later.
What to expect at the first Democratic debate
The storyline of the Democratic presidential contest so far has been the surprisingly successful challenge by democratic socialist Bernie Sanders to frontrunner Hillary Clinton. And this will be the first time they're debating face to face — so the big question is how they'll handle each other.
Sanders, for instance, has tried quite hard to stick to the issues, and has generally avoided media-presented opportunities for him to explicitly attack Clinton. Since this is only the first debate, and things have generally been going well for Sanders's campaign, I expect he'll keep doing that, and emphasize how much he respects Clinton — while implying that he has very different views from her on core issues. One tricky issue for him, though, could be gun control, where he has a less liberal record than Clinton or O'Malley.
Hillary Clinton's debate performances during the 2007-'08 campaign were generally strong, but it's been a long time since then, and she's facing different opponents. Since she's being attacked from the left, expect her to emphasize how much she agrees with Sanders — while also implying that she's more realistic, pragmatic, and electable. The moderators will surely ask about her recent announcement that she opposes the Trans-Pacific Partnership — and her rivals will surely try to portray this position as opportunistic.
Then there are the other candidates — none of whom have had any discernible success in the campaign yet, and all of whom will be eager to make some kind of splash. Expect them all to pile on and attack Clinton.
Martin O'Malley, for instance, likely began this year thinking he'd be getting all the attention as Clinton's strongest challenger, and has been trying hard to position himself to her left. Yet Sanders has gotten all the buzz from progressive activists. So while O'Malley will surely try to contrast himself to Clinton as usual, watch to see if he goes after Sanders too — perhaps emphasizing his own lifelong Democratic roots, in contrast to Sanders's lack of membership in the party. The moderators will likely quiz O'Malley about his record on policing issues in Baltimore, which is controversial on the left due to his support of zero-tolerance policing.
As for Webb and Chafee, who knows? These two lower-profile candidates appear to be less focused on actually winning — and could therefore be loose cannons. According to MSNBC's Alex Seitz-Wald, who reviewed their past debate performances, Webb is "humorless and combative" with "an ability to aggravate opponents," and Chafee is "surprisingly ferocious." Both will probably argue that Clinton is too hawkish on foreign policy.
How to watch
When: 8:30 pm Tuesday
Where: Wynn Las Vegas casino hotel