Atlanta, Georgia (CNN) The Democratic presidential race shifts Wednesday night to the Deep South, as the 10 leading contenders square off on stage with less than 11 weeks left before the Iowa caucuses.

With impeachment hearings underway in Washington, the Democratic field is gathering under a diminished spotlight, with candidates fighting for attention with their other 2020 rival: President Donald Trump.

The race remains wide open, with a four-person top tier: Massachusetts Sen. Elizabeth Warren and Vermont Sen. Bernie Sanders are the progressive favorites; former Vice President Joe Biden retains strong support among the black voters who are especially important in the South Carolina primary and on Super Tuesday; and South Bend, Indiana, Mayor Pete Buttigieg has surged to the top of the polls in Iowa.

It is also unsettled. Former New York City Mayor Mike Bloomberg is weighing a late entry and, if he enters the race, would flood the airwaves and social media networks with advertisements. Former Massachusetts Gov. Deval Patrick has also just entered the fray. Neither will be on the debate stage.

Warren, Sanders, Buttigieg and Biden will be joined on the 10-person stage in Atlanta by California Sen. Kamala Harris, entrepreneur Andrew Yang, New Jersey Sen. Cory Booker, Minnesota Sen. Amy Klobuchar, billionaire investor Tom Steyer and Hawaii Rep. Tulsi Gabbard.

Here are nine things to watch in Wednesday night's debate, which begins at 9 p.m. ET on MSNBC:

Buttigieg's turn in the spotlight

The South Bend mayor has made himself a central figure in past debates. That'll continue with Wednesday night's contest in Atlanta, but -- in the eyes of rival campaigns -- with a much different tone.

Since the field of Democrats last met for a debate in October, Buttigieg has had a burst of momentum, especially in Iowa, where a recent CNN/Des Moines Register poll found him with a sizable lead in the first state to vote in the 2020 election process.

Like Biden initially and later Warren, a rise in the polls brings heightened scrutiny -- and sharper criticism -- on the debate stage.

"Pete definitely has a target set at him with this next debate," said a senior aide for an opposing campaign. "I would think many of the candidates are going to want to talk about their credentials in comparison to Pete."

Among Buttigieg's biggest problems: Though polls show him performing well among white voters, he has struggled to garner any support from African American voters -- a key portion of the Democratic electorate, particularly as the race moves to more diverse states like South Carolina and on Super Tuesday. A Quinnipiac University survey released this week found his support at 0% among black voters in South Carolina.

Obama's admonition and shadow looms large

Once again, former President Barack Obama is looming over the debate -- not only his health care policy and immigration record, as during previous gatherings, but now his gentle scolding for the 2020 Democratic field.

The former president recently injected himself into the race in the biggest way yet, offering no implicit endorsement, but delivering a stern admonition at the campaign's direction.

By imploring candidates to offer plans "rooted in reality" to avoid turning off key swaths of the electorate, Obama was trying to nudge the conversation toward the middle. It's a safe bet that Biden, Buttigieg and Klobuchar will seize on his remarks, while Bernie Sanders and Warren will not.

But the politics of Obama remain complicated and require a careful balance. Criticizing him too sharply is risky, as several candidates learned during the Detroit debate in July.

So will someone confront Obama's words? Don't count on it. He remains far more popular than any of the Democrats on stage.

Biden's southern firewall is tested

Biden has wobbled in Iowa and New Hampshire as Buttigieg and Warren have surged, but his campaign is buoyed by its strong support from black voters in South Carolina -- something that no other candidate has been able to crack.

African American voters are crucial in the Democratic nominating contest, especially in South Carolina -- the last of the first four states to vote -- and in m states that vote in March.

Increasingly, the reality that his rivals have to find a way to cause Biden's southern firewall to crumble is becoming clear.

That means Harris and Booker, in particular, still need to go through Biden. Buttigieg, meanwhile, is desperate for a foothold with black voters.

One issue that could come up Wednesday night: Marijuana. Biden last weekend said more study is needed to determine whether it's a "gateway drug" before he'll support marijuana legalization on the federal level -- something all of his top Democratic rivals support, and many see as a matter of racial justice.

The 'Medicare for All' fight takes a twist

In the last debate, Warren -- still climbing in the polls at that point -- came under a barrage of attacks from the primary's moderate faction over her support for the single-payer health insurance plan popularized by Sanders.

In the days after, Warren was pressed repeatedly to explain how she would finance Medicare for All and, shortly thereafter, released a plan . That, too, came under sharp criticism from candidates like Biden and Buttigieg.

Since then, the state of this increasingly dynamic race has changed. Buttigieg has emerged as the clear leader in Iowa, and Warren, in releasing a Medicare for All transition plan that breaks up its implementation into two separate pieces, has felt some heat from the left.

Still, Warren has not backed off her support for the program and, this time around, it is likely that Buttigieg will face more tough questioning from Warren and others. The South Bend mayor's own proposal, which he calls "Medicare for all who want it," has not yet faced real, sustained scrutiny.

That is going to change on the debate stage in Atlanta.

Will Sanders take on Warren?

Sanders has long treated Warren as an ideological ally. But the new details of Warren's Medicare for All transition and financing plans present an opportunity for Sanders, who argues that Medicare for All should be funded in large part through progressive income taxes, to add some pressure from the left to the heat she routinely gets from more moderate candidates on the issue.

Though Sanders has been extraordinarily careful not to attack Warren, he could also use the debate -- and Warren's recent decision to break up Medicare for All into two legislative steps -- as a reminder to supporters of the policy that he is the only candidate promising to push it forward in its entirely at the beginning of his presidency.

Sanders could also seize on the Biden campaign's dropped opposition to the formation of a super PAC. Sanders has long railed against money in politics -- and now that the former vice president is being bolstered by a super PAC, Sanders could return to his 2016-era "$27"-style messaging to draw distinctions and highlight Biden's reversal.

Voting rights will not be ignored

Democrat Stacey Abrams narrowly fell short of winning last year's governor's race in Georgia, coming within about 55,000 votes of defeating GOP Gov. Brian Kemp.

The campaign was shot through with reports of voter suppression by state Republicans, led by Kemp -- then Georgia's secretary of state -- including his controversial past purging of voter rolls, wonky voting machines and questions over the rejection of absentee ballots.

After 10 days of post-election challenges, Abrams last November acknowledged Kemp as the governor-elect. But in the same speech, she also announced plans to launch a broader lawsuit against the state's "gross mismanagement" of its elections, through a new group called Fair Fight. A number of candidates will spend the day after the debate here at a Fair Fight Action phone bank, where they will help call voters -- more than 300,000 of them -- who are slated to be taken off the rolls.

For those reasons, questions about voting rights are expected to feature on Wednesday night in a way they have not in previous debates, where the issue has largely been lost in the tumult over health care and taxes. With the Democratic candidates -- across the ideological spectrum -- vying for the support of Abrams and Georgia's increasingly influential party leaders, expect a comprehensive onstage conversation, with denunciations of Kemp and Republican efforts nationwide to make voting more difficult.

Kamala Harris looks for a way out of the woods

Kamala Harris's campaign has been in search of rock bottom for months, and hopes a strong performance at Wednesday night's debate could provide proof that the worst for the California senator is behind her.

The stakes are especially high for Harris, who has been mired in the low single digits in polls since the weeks after she surged following an attack on Biden over his record on busing to desegregate schools the first time they shared the stage.

The candidate who was once seen by donors and operatives as having the most potential to win over the broad spectrum of the Democratic electorate in the 2020 field has been forced to shutter her campaign in New Hampshire and move staff from Nevada and California -- two states she once saw as crucial parts of her path to the nomination -- to Iowa in a bid to avoid irrelevance there.

So far, the move has borne little fruit. Harris, according to the CNN/Des Moines Register poll released earlier this week, is at 3% in Iowa, well back from the top tier of candidates and bunched with a host of other competitors.

Harris, during the campaign shakeup, kept her operation in place in South Carolina, viewing it as a state where she could activate black voters -- particularly African American women -- to propel her to victory. But a recent poll found Harris at 3% in the state, far behind Biden, Sanders and Warren and narrowly behind Buttigieg, despite him getting zero support from black voters.

The problem for Harris as she seeks to engineer another marquee moment is that her best one so far -- the clash with Biden -- didn't lead to sustained success. While Harris' fiery confrontation was celebrated immediately after the debate, her heavy-handed follow-up made the moment look manufactured, a view that was furthered when Harris' position on busing days after appeared to be more in line with Biden's views.

Harris has already reached the Democratic National Committee's thresholds to qualify for the December debate, but her precarious position in the polls could mean Wednesday night is her last real chance to generate the momentum she desperately needs.

Amy Klobuchar looks for another moment

Klobuchar is looking for another chance to make a good impression.

At the last debate in Ohio, the Minnesota senator broke through in ways that she previously hadn't, particularly by pressing Warren again and again to explain how she would pay for her Medicare for All plan.

But she is far from the only Democratic rival seizing on that terrain. As Klobuchar looks to capitalize on her strong October debate performance, other candidates may be looking for a Klobuchar moment of their own in Atlanta.

The field's top tier is crowded, so Democrats like Booker and Harris are looking for ways to climb up one rung of the ladder at a time -- as Klobuchar did.

In Iowa, at least, Klobuchar's path is complicated by the rise of Buttigieg. She has two decades more experience under her belt, as a prosecutor and a senator. Will she remind voters of that Wednesday night?

Time has run out for low-polling candidates

Democratic long-shots in a field that once topped 20 candidates for months have leaned on the late-breaking history of presidential nominating battles as a rhetorical crutch: 2004 Democratic nominee John Kerry trailed Howard Dean into late 2007, they point out. Remember that the 2008 Republican standard-bearer, John McCain, was considered a failure in the summer of 2007, they say. Hillary Clinton was still polling ahead of Barack Obama at that point in the 2008 race, too, they note.

The expiration date for those excuses has arrived.

Kerry and McCain's low points were still higher than many of the low-polling Democrats in the 2020 race. And Obama began making his move past Clinton at the 2007 Jefferson-Jackson dinner in Iowa -- an event that's already happened this year.

The race has reached the stage where candidates need serious cash for digital and television advertisements, and to scale up their campaigns to prepare for Super Tuesday in early March.

The candidates will debate again in December, but Wednesday night's showdown is, for many, their last chance to stand out and build serious momentum before it's too late.

Booker is one candidate facing a moment of truth. He's stood out in previous debates, has what's widely regarded as a strong campaign operation and a theoretical path: Black voters, especially in the South, are absolutely crucial to winning the Democratic nomination. But he's mired in the low single digits, and Patrick's entrance into the race was something of a vote of no-confidence.

Steyer has shown he's willing to spend millions of his own dollars on the race -- but it hasn't paid off in anything but low single-digit poll numbers.

Gabbard got a jolt from Republicans and independents who will vote in the Democratic primary when she was lambasted by Clinton. But her attacks against rivals like Harris on the debate stage haven't translated into much more than a couple extra points in the polls.

Even Yang, the entrepreneur and first-time candidate with a sizeable online following who draws large, devoted crowds to events and is emerging as a strong fundraiser, has to give more Democratic voters a reason to take him seriously as their potential standard-bearer -- something he's attempting to do, as he focuses more on telling personal stories of how people are benefitting from a small pilot program he's running for his campaign's animating proposal: a $1,000-a-month "freedom dividend" for all Americans.