Qualifying for the first Democratic presidential debate was the easy part. Now comes the challenge of preparing for it.

With the debate in Miami less than a month away, at least a half-dozen major candidates have begun to block out time or lighten their schedules to prepare.

On telephone calls and in conference rooms, advisers are peppering them with potential questions. The candidates are practicing tightening their answers, cognizant of the 7 to 10 minutes of total speaking time they expect to be allotted. And they are watching clips of the 2016 Republican presidential primary debates to familiarize themselves with the dynamics of debating on a crowded stage.

All of it is taking place under the expectation that the first debate will represent the most significant milepost of the campaign to date, a make-or-break event that likely will lead to the first winnowing of the crowded 23-candidate primary.

For candidates accustomed to far more control over their circumstances, the run-up is proving to be an unsettling experience.

“This is not a scenario that any of them have been in,” said Philippe Reines, a longtime Hillary Clinton confidant who played the role of then-candidate Donald Trump in Clinton’s debate preparations in 2016. “It’s almost like a particle accelerator. … It becomes a group dynamic that you can’t really control.”

Few candidates, if any, have debated nine others at once. None of the contenders knows yet whether they will appear on June 26 or 27 — as many as 20 candidates will be split over two debates on successive nights — or even whom they will be debating against. Those dynamics are combining to create a deep sense of uncertainty and frustration surrounding evenings likely to be marked by the campaign’s largest national viewership yet.

One campaign adviser to one top-tier campaign likened the unknowns surrounding the debate to a “black box.” An adviser to a different campaign said: “It definitely hurts how one prepares for a debate, especially since it’s your first introduction on the national stage. … It’s tough when there is a ton of pressure to meet thresholds, and we can’t get more than two weeks to know who we will be on stage with.”

The campaigns are expected to get answers about debate logistics and format on a conference call on Thursday with Democratic National Committee officials.

But the candidates are unlikely to know on which night they will appear until about two weeks before the debate — and some candidates will continue to sweat out whether they will qualify at all.

In interviews with about a dozen campaigns, advisers said they are beginning to intensify their preparations. Several officials said they would hold campaign run-throughs with people acting as proxies for other candidates, though perhaps not with a full complement of nine actors representing nine candidates. Others are trying to anticipate from which opponents they might expect crossfire.

Mayor Pete Buttigieg of South Bend, Ind., and Sen. Kamala Harris of California are among those who recently started focusing on debate preparations. An aide to Sen. Elizabeth Warren of Massachusetts told POLITICO that while it’s still too early to intensely strategize for the debate, it’s a challenge “when we don’t know who we will be on stage with until a few days out” and that it would be “hard to have a moment” with 10 people competing for airtime.

Another campaign highlighted the difficulty of not knowing who all would share the stage, saying staffers who would otherwise study old debate tapes of potential competitors to “get in their heads” are instead presenting a composite of challengers in practice sessions.

"We’re preparing for a cross-section; there are a lot of candidates who have similar positions on the big issues. We know where a lot of our peers are coming from,” said an adviser to California Rep. Eric Swalwell.

For lesser-known candidates such as Swalwell, the debates present an opportunity to gain a much-needed step in the primary.

Washington Gov. Jay Inslee has hired a debate director, Geoff Potter, and he and his advisers have held initial calls about how to approach the first debate. Unlike a congressional, gubernatorial or Senate debate — where the resulting media coverage can be more significant than the debate itself — millions of viewers are expected to be watching next month, amplifying the significance of a live performance.

On a recent call among Inslee’s advisers about the debates, former Florida Gov. Jeb Bush’s criticism of Trump as a “chaos candidate” in 2016 came up as an example of how one-liners can fall flat. Several other candidates have made similar calculations, with one adviser to a lower-tier candidate saying, “You can’t be artificial about this. … Voters see through it when candidates deliver a zinger.”

John Delaney, a former congressman from Maryland, has been watching recordings of both the Democratic and Republican primary debates in 2016. His communications director, Will McDonald, said, “For most people, this is going to be the first or second time they’re tuning into this contest.”

Advisers for campaigns at the bottom of polls said part of their focus in the coming weeks will be navigating how lesser-known candidates can distinguish themselves without coming across as combative.

Advisers to multiple candidates suggested that they are preparing to draw contrasts in the debate — but more likely with the Democratic field as a whole, not any candidate in particular.

“Maybe you have three minutes in a debate, four minutes; what are you going to do? Are you going to try to swing for the fences, the way [Marco] Rubio did going after Trump? Or are you going to attack somebody else?” asked Stuart Stevens, the chief strategist for Republican Mitt Romney’s 2012 presidential campaign. “It’s just an impossible situation. … The Lincoln-Douglas debates, it wasn’t Lincoln, Douglas, Johnson, Smith, Harris, Fitzgerald, O’Reilly. It was Lincoln-Douglas.”

Lamenting the format, Stevens said the debate will not resemble a debate as much as a “multicandidate press conference.”

“It’s crazy,” he said.

One rival everyone is preparing for — and who will not appear on stage — is Trump. He’s widely expected to tweet his reactions to the debates as they unfold, and Democratic strategists said it’s possible the moderators will ask questions based on his tweets. For Democrats who have benefited politically from provoking the president, the possibility of engaging with him in real time is widely viewed as a potential boon.

But several candidates are also preparing to defend themselves against criticism of their records. That could come from other candidates — but more likely, their advisers believe, in the form of challenging prompts from the moderators. Partly for that reason, several campaigns pointed to the proliferation of televised town halls as their most significant practice for the first debate.

“I think preparing for a debate is just like preparing for a town hall,” Sen. Kirsten Gillibrand of New York told POLITICO recently. “You just want to be able to articulate what you’re for in a way that is concise, which is harder for me, and direct.”

Asked if she had been drilled on giving shorter answers by her staff, she said, “Not yet.”

“But I did practice it in my first town hall with MSNBC because every break we had, the producer would say, ‘OK, those were great answers, but shorter, shorter, shorter.’ I was like, ‘OK, OK, OK!’" she added. "So I think for my second town hall, CNN, I think, if you watched it, you’d see that the answers were shorter. So I’m getting better.”

Several advisers to candidates outside the top tier said they hope their candidates are onstage with former Vice President Joe Biden or Sen. Bernie Sanders of Vermont — the top two polling contenders — either to guarantee they’ll get the most eyeballs or convey an immediate contrast. One adviser of a second-tier candidate relished the thought of their candidate being onstage with Biden: “I can offer you the younger alternative of what you’re looking at right now.”

“I want to be next to Joe Biden so the country can Google, ‘Who’s the Asian man next to Joe Biden,’ and then they will discover Andrew Yang,” little-known entrepreneur Yang said. “I think that’s the ideal. I’ve done the math, and I have approximately an 8 percent chance of standing next to Joe Biden.”

Sen. Amy Klobuchar of Minnesota told reporters in New Hampshire recently, “It’s going to be really interesting because you’re going to have so many people, and it’s luck of the draw which night you’re going to be, who you’re going to be standing next to — maybe going to be really tall, don’t know.”

The front-runners are feeling pressure in a different way than those still trying to make an introduction to a national audience are. With so much attention on their ages, Sanders and Biden cannot have a Rick Perry-like forgetful moment. Biden will be expected to be the adult in the room. And Warren’s reputation as a policy wonk means she’ll have added expectations to drive the discussion or at least have well-thought-out responses to most questions.

As the front-runner, Biden is widely considered the most likely to face a pointed challenge from his competitors in the debate. But advisers to many of his opponents remain wary of attacking him too sharply. Even if a competitor could wound Biden in the debate, in a multicandidate field, it is unclear who would benefit.

“In 2016, if Bernie attacked Hillary and he landed a blow, it’s possible those people came to Bernie,” Reines said. “And if Hillary landed a blow on Bernie, people came to her. Now, you have a situation where [Elizabeth] Warren might take a shot at Bernie Sanders, because they’re kind of fighting for the same pool, and a voter says, ‘I don’t like Bernie’s answer, but I don’t like the way Warren asked it, so I’m going to take a hard look at Kamala.’”

He said, “You’ve got a little bit of a whack-a-mole situation, and that’s going to play out onstage.”

Christopher Cadelago contributed to this report