Here’s what you need to know:
The candidates on stage tonight: Bill de Blasio, Tim Ryan, Julián Castro, Cory Booker, Elizabeth Warren, Beto O’Rourke, Amy Klobuchar, Tulsi Gabbard, Jay Inslee and John Delaney.
Who needs the biggest boost? Literally.
One short question ahead of the debate: Will any of the candidates opt to stand on an “apple box” to boost their height?
After all, Mr. de Blasio will be standing 6-foot-5 and seven-eighths of an inch high at one edge of the stage.
A memo from the Democratic National Committee to the campaigns ahead of the debate said that two sizes would be available. But it’s not clear that the extra inches could be worth the potential for ridicule.
Dan Pfeiffer, a Democratic strategist, recalled working for Senator Tom Daschle in 2004 and having him sit on a pillow during a debate to appear taller. It became a big story.
“Lesson: looking short would have been better,” Mr. Pfeiffer wrote on Twitter.
How should candidates stand out in a crowded debate? Look to 2016.
Democratic presidential hopefuls face the daunting task of vying for attention in a series of debates including 20 candidates (Thursday night, the next set of candidates will debate one another). Republicans had a similar problem in 2016.
For the first time, more than one woman will be in a presidential campaign debate.
Hillary Clinton competed in two cycles of presidential primary debates, in 2016 and 2008, and she was the only woman on stage both times. Most presidential campaigns since 2000 have had a female candidate running in one or both parties — until this year, when a record six women entered the race for the Democratic nomination.
So Wednesday night’s debate will make history: instead of one woman on the presidential debate stage, there will be three: Ms. Warren, Ms. Klobuchar and Ms. Gabbard.
Some Democratic activists and elected officials have expressed concerns that the women running for president are being overlooked by many voters and some donors, pointing to the fact that Joe Biden and Bernie Sanders have been leading in the polls and raising significant money.
The debates are the biggest chance yet for the six women to make the most a national spotlight. Many of them have significant legislative and debate experience, and their supporters hope that their resumes and some of their personal characteristics that have received less attention — Ms. Klobuchar’s sense of humor, for example — may attract notice as well.
Thursday night’s debate will include Kamala Harris, Kirsten Gillibrand and Marianne Williamson.
Warren visits center for migrant children.
Hours before taking the debate stage, Ms. Warren traveled to Homestead, a South Florida facility for unaccompanied migrant children that is often referred to as a detention center.
Homestead has become a symbol, in some political and immigration activist circles, of the Trump administration’s hard-line immigration policies, which once officially included separating children from their families at the U.S.-Mexico border.
Ms. Warren climbed up on a stepladder to wave to children inside the facility.
“These were children who were being marched like little soldiers, like little prisoners, from one place to another,” she told reporters.
She is one of several presidential candidates to pledge to visit the facility.
Ms. Klobuchar and Jane Sanders, the wife of Bernie Sanders, also visited later Wednesday. Mr. Swalwell was there earlier this week and Mr. O’Rourke has said he will visit Thursday, and a host of contenders, including Kamala Harris and Pete Buttigieg, are expected there Friday.
Patricia Mazzei contributed reporting from Homestead, Fla.
What’s happening on the ground in Miami?
How many candidates are here in Florida for the first Democratic debates? So many that one, former Gov. John Hickenlooper of Colorado, was spotted on Wednesday getting asked about whether he was picking up press credentials.
“I’m a candidate,” replied Mr. Hickenlooper, according to NPR’s Scott Detrow who was there. (Something similar happened to Republican presidential candidate Rand Paul in 2015.)
Candidates were nearly crossing paths inside the Adrienne Arsht Center in Miami. As Senator Elizabeth Warren was doing her stage walk-through, Senator Bernie Sanders was high in the rafters taping an interview on MSNBC. (Their aides tweeted photos at one another.)
While most candidates were laying relatively low before the debate, Mr. Sanders had a combative response to a question about whether he would leave the race before the Democratic convention if it was clear he would not be the nominee.
“I intend to be the Democratic nominee,” he replied. But as an NBC correspondent pressed him on complaints from “some people” that his prolonged candidacy hurt Hillary Clinton, he retorted, “‘Some people’ say that if, maybe, the system was not rigged against, I would have won the nomination and defeated Donald Trump. That’s what ‘some people’ say.”
“So I think,” Mr. Sanders added, “we’re going to play it out.”
Will Warren hit her rivals in absentia?
Ms. Warren has gained on Mr. Sanders in some polls as both fight for the mantle of progressive standard-bearer. And in recent weeks, Mr. Sanders and his allies have appeared to take some oblique swipes at her.
Ms. Warren will be onstage Wednesday while Mr. Sanders won’t appear until Thursday — but it is worth watching whether Ms. Warren, directly or implicitly, proactively moves to draw contrasts with Mr. Sanders.
She has been more open in her differences with former Vice President Joseph R. Biden Jr., criticizing his high-dollar fund-raising and his references last week to working with segregationists during his time in the Senate.
Mr. Biden won’t be onstage with Ms. Warren either, but it will be notable if she sharpens her argument against him on Wednesday.
Telemundo is co-hosting the debate.
Telemundo will be the first Spanish-language network to co-host a Democratic presidential debate, bringing the candidates to thousands of Latino living rooms across the country and bringing Latinos’ concerns to prime time.
If there is a moment to address Latino voters, it’s this one. Moderators and candidates might frame their questions and responses to address the concerns of the Latino community a lot more than usual.
Immigration and the border and family separations are sure to come up. Health care and job security — Latino voter’s biggest concerns is likely to come up as well. Spanish-sprinkled answers are to be expected, especially from Mr. Castro and Mr. O’Rourke.
How is Twitter talking about the debate?
The debate will not just occur live on national television. It will be discussed, digested, and shaped simultaneously on the second screen of Twitter (#DemDebate is the official hashtag).
And every campaign knows it.
Agile campaigns these days write numerous tweets, both for themselves as campaign officials and for influential outsiders who might amplify their message or big moment.
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