Des Moines, Iowa (CNN) The final debate before the Iowa caucuses brings new challenges for the Democratic presidential field .

In the Middle East, a crisis is rising, with President Donald Trump increasingly at odds with Iran . In Washington, the looming Senate's impeachment trial could soon command the national spotlight.

Polls show a tightly bunched top tier ahead of the February 3 caucuses, with Vermont Sen. Bernie Sanders , Massachusetts Sen. Elizabeth Warren , former South Bend, Indiana, Mayor Pete Buttigieg and former Vice President Joe Biden all with real shots at victory.

That makes Tuesday night's debate, which will have the smallest number of candidates onstage to date, a high-risk, high-reward affair -- with each candidate forced to weigh whether they want or need the sorts of direct confrontations a smaller six-person stage could bring.

The debate also includes two candidates still vying for a breakout: Minnesota Sen. Amy Klobuchar , whose hopes hinge on a stronger-than-expected performance in Iowa, and billionaire investor Tom Steyer , who qualified for the debate on the strength of his polling in Nevada and South Carolina, the third and fourth states to vote. In those states, television ads aired by Steyer's campaign and former New York City Mayor Michael Bloomberg have dominated the airwaves. (Bloomberg will not be on stage Tuesday.)

Here are nine things to watch Tuesday night in Des Moines:

Sanders and Warren fight breaks out

The months-long, unspoken pact between the field's two most progressive candidates -- Sanders and Warren -- finally appears to have broken.

On Monday, CNN reported that in a 2018 meeting between the two, Sanders told Warren he believed a woman could not win the presidency, according to four sources.

Sanders disputed the account.

"It is ludicrous to believe that at the same meeting where Elizabeth Warren told me she was going to run for president, I would tell her that a woman couldn't win," Sanders said in a statement to CNN.

But on Monday evening, Warren released a statement backing up the sources' characterization of the meeting.

"I thought a woman could win; he disagreed," she said.

It's no surprise that the two would eventually be at odds. Sanders has increasingly consolidated the party's left wing, while Warren is a favorite of college-educated liberals. But both likely would need broad support from progressives to win the nomination.

Over the weekend, Politico reported on the Sanders campaign's talking points instructing volunteers to say that Warren's supporters are "highly-educated, more affluent people who are going to show up and vote Democratic no matter what" and that "she's bringing no new bases into the Democratic Party."

Warren told reporters that she was "disappointed to hear that Bernie is sending his volunteers out to trash me." Sanders himself appeared to distance himself from the memo, saying on Sunday that his campaign has "hundreds of employees" and that "people sometimes say things that they shouldn't."

"I have never said a negative word about Elizabeth Warren, who is a friend of mine," Sanders said.

This debate is different on foreign policy

Trump's decision to order the strike that killed a senior Iranian military commander gave rise to a new crisis in the Middle East -- and is forcing Democratic presidential contenders for the first time to seriously detail their own views of foreign policy and the United States' role in the world.

Biden's campaign is convinced this new reality helps the former vice president the most: He has decades of foreign policy experience and worked alongside former President Barack Obama to craft international deals like the Iran nuclear agreement, which Trump scrapped. He has long argued he's the candidate best prepared to work effectively on the world stage immediately after taking office, and -- in an effort to draw contrast with Trump -- is emphasizing the stability he would offer in television ads, statements and campaign appearances.

But Sanders sees an opening, too: He's lambasted Biden for voting in 2002 to give then-President George W. Bush authority to use military force in Iraq. It's the issue that helped propel Obama past Hillary Clinton in the 2008 Democratic primary, and could retain some of its potency 12 years later.

Meanwhile, Buttigieg -- a US Navy Reserves veteran who served one tour in Afghanistan in 2014 -- is emphasizing his own military credentials. He introduces himself "as a veteran" in a new Iowa TV spot.

The all-white stage

What started as the most diverse presidential primary field in history has turned into an all-white debate stage, frustrating Democratic activists who worry that the diversity of the party's base isn't being represented in the 2020 race.

This month's lineup is the same as December's, minus Andrew Yang -- the Asian American entrepreneur who'd said on stage last month it was "both an honor and disappointment to be the lone candidate of color." Yang missed the polling requirements to qualify for the January debate.

Meanwhile, New Jersey Sen. Cory Booker -- who had missed the December debate, too -- dropped out of the presidential race on Monday after again failing to qualify for the debate stage. California Sen. Kamala Harris and former Housing and Urban Development Secretary Julián Castro have also dropped out.

Impeachment looms

For three of Tuesday night's participants -- Sanders, Warren and Klobuchar -- opportunities to visit Iowa ahead of the February 3 caucuses might soon be running out.

The looming impeachment trial could force members of the Senate -- including Michael Bennet, who did not qualify for the Iowa debate -- off the campaign trail and back to Washington at an inconvenient time.

Even more importantly for the candidates: It could knock the 2020 race off the front pages as the Republican-led Senate's handling of the Democratic-controlled House's move to remove Trump from office takes center stage.

That means fewer opportunities in the final three weeks before the Iowa caucuses, and final four weeks before the New Hampshire primary, to shake up the race -- a reality that only raises the stakes in Tuesday night's debate.

Bernie under the microscope

Sanders proved that he has a loyal coalition of supporters on the left in 2016. But in the 2020 race, he has largely avoided scrutiny from his opponents. At times, he's even being credited by them in underhanded attempts to whack other rivals, such as when Biden said that, in contrast to Warren and Harris on increasing taxes to pay for their health care plans, Sanders is "honest about it."

That may change on Tuesday, after the recent CNN/Des Moines Register poll found Sanders was on the rise as he polled at 20% among likely Democratic caucusgoers -- a hint that the Sanders campaign may be picking up momentum at just the right time.

Sanders' campaign has grown feistier in the weeks leading into this debate, too, especially in taking on Biden on foreign policy and his vote for the North American Free Trade Agreement.

Sanders was believed to have a high floor in Iowa, meaning that he came into the contest with a core group of supporters -- largely from last cycle -- who were devoted to him and unlikely to move to another candidate. What the CNN poll and on the ground reporting have shown, though, is that his support may be expanding beyond that base as the February 3 caucuses approach.

That dynamic may also lead candidates like Buttigieg and Biden, both of whom have stark ideological differences with the senator and high hopes in Iowa, to take him on at Tuesday night's contest, a dynamic that could be helped by the fact that there will only be six candidates on the stage.

Buttigieg needs to stop his Iowa slide

Buttigieg's high point in Iowa came in early November when a CNN/Des Moines Register poll found him in the lead: 25% of likely Democratic caucusgoers supported his once long-shot presidential campaign.

And for nearly two months, Buttigieg rode that poll -- and the belief it created among voters, pundits and activists that he was among the front-runners in Iowa -- as he crisscrossed the state.

Then, on Friday, the latest CNN/Des Moines Register poll found Buttigieg had slipped 9 percentage points to 16%, solidly in the middle of a four-person top tier with Biden, Warren and Sanders.

This debate is Buttigieg's last chance to stop that slide and deliver a debate performance that reminds voters in Iowa why they had gravitated toward him.

Buttigieg took a healthy amount of incoming attacks at last month's debate in Los Angeles -- on his experience, his reliance on big donors and a host of positions he holds -- and everything points to the same happening on Tuesday night. How the mayor handles those attacks could go a long way to defining his Iowa campaign.

But, for Buttigieg in particular, the impact may be broader. Buttigieg, arguably more than any other candidate in the top tier, needs a strong finish in either Iowa or New Hampshire to propel him past the first two states. A falter in this debate could prove disastrous.

Can tonight be different for Klobuchar?

Klobuchar, in particular, needs a breakout night. Iowa's caucus rules require candidates to reach 15% at individual voting sites in order to amass delegates -- and polls have shown her well below that threshold, putting her at real risk of a distant fifth-place finish that would all but end her campaign.

Klobuchar has performed well in debates previously, but has so far struggled to use those performances to launch her candidacy into the top tier of Democratic candidates in Iowa.

Klobuchar earned 6% support in Iowa in the latest CNN/Des Moines Register/Mediacom poll, far behind the four candidates leading the Democratic race.

Undecided Iowa

Barbara Wells, a 62-year old social worker from Des Moines, had a busy day on Sunday. She woke up and drove straight to a Warren event in Marshalltown, then took in a Klobuchar town hall in Perry and ended her day with a Buttigieg town hall back in Des Moines.

Wells remains undecided -- like many voters here in Iowa.

That fluidity means that while polls show a tight race between Sanders, Warren, Buttigieg and Biden, the next three weeks could bring wild swings in support as Iowans like Wells begin to make their final decision.

"We circled around and I watched no football today," Wells joked as she waited for Buttigieg to speak.

"One thing that really impresses me about Amy is she talks about how she worked with this person to get this done and this person to get that done. And so that's where I think she really has a lot of experience that causes me to give Pete a second look," Wells said.

Campaigns are well aware of voters like Wells -- the people who have a list of three candidates they are willing to support and likely won't decide on who until days before the caucus.

Tuesday night's debate will be a key moment for these voters, too, and the best chance for a candidate to move up in that top three list or eliminate other candidates from a voter's list.

Tom Steyer will also be on stage

The billionaire former hedge fund manager who has spent more than $100 million on television ads was underwhelming in the last debate, largely drowned out by the more experienced debaters on stage.

Steyer barely made Tuesday's debate -- he got in with a pair of Fox News polls, one from Nevada that found him at 12% and another from South Carolina that found him at 15% -- but the question for Steyer and his campaign is whether he can make the appearance count.