WASHINGTON — Tuesday marks the last time the leading Democratic presidential contenders will face a national audience on the debate stage before primary voting begins, making it the most significant moment of the 2020 primary season to date.

The prime-time faceoff comes just 20 days before Iowa’s caucuses as polls suggest the nomination is truly up for grabs. Heading in, there were new signs of tension among all the top-tier candidates, particularly Bernie Sanders and Elizabeth Warren. If ever there was a time for the six contenders on stage to take the gloves off, this is it.

Six big questions heading into the Des Moines debate, to be carried live on CNN:

IS THE DEMOCRATIC PRIMARY ABOUT TO EXPLODE?

The conditions are ripe for a full-blown political brawl in Iowa. Less than three weeks before voting begins, there is no front-runner, which means that every candidate on stage has the opportunity to stand out. And as Cory Booker found out, it’s hard to stand out in 2020 playing the nice guy. Joe Biden and Pete Buttigieg have already given and taken their fair share of heat in past debates. And suddenly, Warren and Sanders are sparring with each other. It’s always risky to go on the attack, but with time running out and tensions rising, it’s easy to envision fireworks.

A DEBATE OVER GENDER?

A dust-up between Warren and Sanders is moving gender to the center of the debate. Sanders was on the defensive Monday facing anonymous reports that he told Warren during a private meeting in 2018 that a female candidate couldn’t beat President Donald Trump. Warren confirmed the report in a statement Monday night, while Sanders has vigorously denied it. The dispute reflects Sanders’ growing stature in the field. But it also speaks to the bigger role women will play in choosing the next president — and it underscores anxiety among many Democrats that voters aren’t ready for a woman in the White House. While many Democrats say they want a female president, they say the top priority is beating Trump in November.

IS IT BERNIE’S TURN FOR THE FRONT-RUNNER TREATMENT?

His team insists he’s well positioned to win Iowa and New Hampshire, so perhaps it’s fitting that Sanders is looking like a top target heading into the debate. On Monday, he was on the defensive facing anonymous reports that he told Warren during a private meeting in 2018 that a female candidate couldn’t beat Trump. Warren confirmed the report in a statement Monday night, while Sanders has vigorously denied it. That was just days after news leaked that the Sanders campaign was privately encouraging volunteers to play up concerns about Warren’s appeal beyond educated voters. Given her recent struggle for momentum and Sanders’ rise, this is a fight that Warren wants and needs. And don’t forget that Biden and his allies have raised consistent concerns about Sanders’ signature policies and his status as a self-described democratic socialist. Sanders has been an afterthought in past debates, but he enters this one as a central player.

IS THE WINE CAVE CLOSED?

Pete Buttigieg came under heavy scrutiny in the last debate for hosting wealthy donors at a California “wine cave.” While we don’t know if he has visited any other wine caves since, we do know that he’s maintained an aggressive fundraising schedule that relies heavily on big donors. Fundraising represents a defining issue in the broader fight between opposing wings in the Democratic Party: One insists that any money from big-dollar donors is tainted, while the other insists they need all the money they can get to defeat Trump. Given his well-documented struggles with black voters, Buttigieg cannot afford to alienate many progressives, which is exactly what happened around the wine cave.

HOW DOES THE ALL-WHITE FIELD PLAY?

The Iowa debate features the smallest group in the Democrats’ 2020 primary season — and it’ll also be the whitest. Not a single person of color met the Democratic National Committee’s rising donor and polling thresholds. That’s not good news for a party that needs to generate strong minority turnout to defeat Trump, who is tremendously unpopular with black and brown voters, but is making an aggressive case that the Democratic Party is taking them for granted. Tuesday’s lineup may help bolster Trump’s claims, forcing the all-white cast to defend their party on a sensitive issue.

CAN BIDEN MOVE PAST IRAQ?

Last week’s military clash with Iran shines a new spotlight on foreign policy — and Biden. The former Delaware senator and two-term vice president has struggled for almost two decades to explain his 2002 vote to authorize military action against Iraq. Foreign policy should be a strength for him. But the Iraq vote, among other questionable decisions, gives his rivals legitimate lines of attack. Based on recent history, expect Buttigieg and Sanders to be the most aggressive on Iraq. Biden hasn’t helped himself recently by inaccurately describing his position. Few issues illustrate the difference between Biden and his competitors more than this one. Biden knows it’s coming — can he finally move past it?

CAN ANYONE ELSE OUTSHINE THE BIG FOUR?

In a race packed with this much uncertainty, it’s a mistake to dismiss anyone on stage. That includes Minnesota Sen. Amy Klobuchar and billionaire activist Tom Steyer, who may have struggled for attention for much of the last year but have each shown upside in recent weeks. Klobuchar, who had a strong showing in last month’s debate, has a biography made for Iowa. And Steyer, who is flooding early voting states with a massive advertising campaign, raised eyebrows last week with surprisingly strong polling. They have yet to crack the top tier, but with the eyes of the political world on them Tuesday, the opportunity is there.