Houston, Texas (CNN) All of the top contenders in the 2020 Democratic presidential race will be on the same stage for the first time Thursday night when they meet for the party's third primary debate.

In this round, the requirements to qualify were higher -- which winnowed what had been 20-candidate field spread over two nights down to a single, 10-candidate stage.

At the center of the stage, the front-runner in national polls, former Vice President Joe Biden, will be sandwiched between his two chief progressive foes in Massachusetts Sen. Elizabeth Warren and Vermont Sen. Bernie Sanders.

Meanwhile, seven other candidates mired in the single digits in polls -- California Sen. Kamala Harris, South Bend, Indiana, Mayor Pete Buttigieg, New Jersey Sen. Cory Booker, former Texas Rep. Beto O'Rourke, entrepreneur Andrew Yang, former Housing and Urban Development Secretary Julián Castro and Minnesota Sen. Amy Klobuchar -- will all look for ways to break out, with the first states set to vote in the 2020 primary in less than five months.

Here are six things to watch Thursday night in Houston:

1. Biden vs. Warren

These two -- who The Washington Post characterized as long-time frenemies -- have risen to the top of the polls with drastically different strategies, as Biden promises a return to pre-Donald Trump normalcy and Warren argues for bigger, structural changes.

The two have found themselves on opposing sides of economic debates as early as 2005, when Warren, then a professor, opposed a Biden-backed bankruptcy bill in committee. "You gave me hell," Biden told her in 2013, when he swore Warren in to the Senate.

Thursday will be the first time they'll share a debate stage -- a reality that could offer Democratic voters a stark view of the contrasts in how the two approach politics.

Will they engage much directly? It's hard to predict. Advisers for both candidates have been downplaying the notion of a head-to-head battle, though some sniping has emerged, such as a Biden adviser telling CNN the former vice president will argue on stage that "we need more than plans."

As for Warren, a campaign aide said, "She will identify what's broken in America. She will show her plans on how to make real change. And she will talk about how she's building a movement to make it happen."

2. Where does Bernie Sanders fit in?

Polls show that Sanders is right there with Biden and Warren atop the field. And his campaign has howled that the political press is writing him out of stories about the state of the race, even though he is firmly entrenched in the top tier.

But part of why Sanders sometimes doesn't get much mention is that his message and his platform -- "Medicare for All," free college tuition, a $15-an-hour minimum wage -- haven't changed much since 2016, and don't really change based on the venue.

Where will Sanders jump into the fray? One possibility is if Biden repeats his often-used line from his stump speech and early advertising that characterizes the progressive push for Medicare for All as an effort to undo Obamacare.

"I think if that's the conversation that comes up, there'll be no flinch on his part. This is exactly the debate we want to have," Sanders campaign manager Faiz Shakir said.

3. The dangers of punching up

The most memorable moment of the first two Democratic debates was Harris eviscerating Biden over his previous opposition to federally mandated school busing. And it gave Harris a temporary jolt in the polls, sending her briefly into the high teens.

Since then, though, she has fallen back in the pack. And headed into the third debate, Harris' campaign is suggesting the risk isn't worth repeating.

The former California attorney general's new approach: Direct her prosecutorial-style ire toward Trump.

"Kamala will take on Donald Trump directly and will focus on bringing the country together by defeating him and unifying Americans around solutions to our common challenges. She'll make the connection between his hatred and division and our inability to get things done for the country," Harris spokesman Ian Sams said.

The change in approach for Harris shows the challenge facing everyone outside the top three of Biden, Warren and Sanders.

To rise in the polls, Democrats need the candidates ahead of them to fall. But so far, going on the attack hasn't been rewarded in the long term. Harris slipped in the polls. Castro and New York City Mayor Bill de Blasio unloaded on O'Rourke in the first debate, yet still trail him. Former Maryland Rep. John Delaney was central to the second debate, used as a foil for Warren and Sanders -- but he didn't get any traction out of it.

4. How will the low-polling candidates stand out?

Going on the attack hasn't worked, but that doesn't mean some Democrats won't be willing to try it.

Still, most are looking for other ways to stand out.

Buttigieg has cast his campaign as shifting into a new phase, with field offices opening and more hires being made in the early states, as he makes the case for generational change. He's also criticized Trump for demeaning the presidency. "The purpose of the presidency is not to glorify the president, it is to unify the people," he said Tuesday in Washington.

O'Rourke, since the El Paso, Texas, mass shooting last month, has centered his campaign around a push for gun control -- including a call for a mandatory buyback of assault-style firearms. He's also going to Houston, the capital of the energy industry, days after calling for a ban on offshore drilling.

Castro got to Houston early, telling a crowd Monday that the city is "a place where people from different walks of life and all over the world have come to make a life to reach for their American dream. This is the kind of place that rejects Donald Trump."

Klobuchar, meanwhile, told The New York Times she is tired of debate questions to her being framed around her opposition to progressive contenders' ideas -- placing her in the position of telling Democratic voters that they can't have the things they aspire to.

5. What's Yang planning?

Yang has a big idea -- giving every American $1,000 per month -- and a devoted online following that calls itself the "Yang Gang."

But he hasn't made a mark in Democratic debates so far.

Will that change? Well, his campaign manager made an ominous promise. The Daily Beast's Sam Stein tweeted: "Andrew Yang's campaign manager just called to tell me that at tomorrow night's debate, Yang will be doing "something no presidential candidate has ever done before in history." He declined to go further than that."

6. Is one night better than two?

Just 10 candidates qualified for this debate. But it looks like an 11th, billionaire investor and Democratic activist Tom Steyer, will meet the Democratic National Committee's qualifying threshold for the October round.

That could make for a much different setting than we've seen so far -- with five or six candidates on stage spread over two nights.

Would giving fewer candidates more time to express their ideas make for a more informative viewing experience? Or do Democratic voters prefer to tune in and see all their leading candidates at once?

The uncertainty could factor into how each contender approaches Thursday night, since there's no guarantee this same cast of candidates will share the stage again next month -- and therefore no guarantee they'll have the same opportunity to highlight contrasts in policy and approach with the foes they believe they need to knock down again in a few weeks.