WASHINGTON (Reuters) - Six Democratic presidential contenders on Tuesday face perhaps the most pressure-packed debate yet, with voters in Iowa set to kick off the 2020 nominating contest in just weeks.

Slideshow ( 2 images )

The Feb. 3 caucuses set the tone for the primary race and often dictate whether a campaign is viable going forward into other early voting states such as New Hampshire and South Carolina. Polls show the race in Iowa is tight.

Here’s a look at the stakes for the candidates on stage in Des Moines:

JOE BIDEN

National polls still land the former vice president atop the 13-member Democratic field, and recent state polls have him more competitive in Iowa than he had been. The heightened tensions between the United States and Iran have played to Biden’s argument that his deep experience would make him a steady hand in the wake of the global turmoil brought about by volatile U.S. foreign policy. Expect him to tout his national security credentials. Biden may see more incoming fire than in previous debates, as his rivals look to soften his lead before voting begins. So far, that hasn’t worked.

BERNIE SANDERS

Call it the Bernie-assance. Seemingly written off months ago after a heart attack and becoming a progressive afterthought to then-rising Elizabeth Warren, Sanders has gathered strength and set himself up as the top threat to Biden for the nomination. The U.S. senator from Vermont is rolling in money and promises to be in the race for the long run. His challenge on Tuesday will be to show skeptics that he is more than a niche candidate with a fervent following. At some point, Sanders has to make a case for himself as the nation’s chief executive and more than a protest vote against the establishment.

PETE BUTTIGIEG

The most intriguing story of the 2020 race might be that the 37-year-old gay former mayor of a small Indiana city remains on track to finish strong in both Iowa and New Hampshire - and make a real run at the nomination. So far, ceaseless attacks from the party’s progressive wing have failed to slow his stride. That could change on Tuesday, when both Sanders and Warren may feel a more urgent need to take Buttigieg down. While Buttigieg so far has parried effectively, voters likely will be watching more closely than ever to see if he has the chops to leap from the South Bend mayor’s office to the White House’s Oval Office.

ELIZABETH WARREN

Perhaps no candidate on the stage will be under more pressure than Warren. After peaking as the perceived front-runner last fall, she has tumbled to a step below the top-tier candidates. Warren needs a strong showing in Iowa to avoid heading to a showdown in New Hampshire in a weakened position. That may mean the Massachusetts senator will have to abandon her practiced “I have a plan for that” approach and make a more forceful case that she is the best candidate to take on Trump in November. The conflict with Iran offers Warren a chance to make a case for her national security skills in a more prominent way. And she may need to come up with a stronger rebuttal to what critics such as Biden argue – that she is more interested in picking fights with Democratic moderates than uniting the party.

AMY KLOBUCHAR

Klobuchar, a U.S. senator from neighboring Minnesota, has put it all on the line in Iowa. A poor finish there would doom her candidacy. While she earns positive reviews on the trail, has shown herself to be a steady fundraiser and earns praise from the pundit class, Klobuchar has yet to truly break out as a player in the nominating race. Her moderate stances place her in competition with Biden and Buttigieg, but her campaign also sees her as a rival to Warren for the votes of women. It could make sense for her to go after Warren on issues such as healthcare to differentiate herself. She will likely talk a lot about her electoral strength in the Midwest. Plus, she’ll probably make the best jokes.

TOM STEYER

Steyer, a billionaire from California who has swamped the airwaves in Iowa and elsewhere with ads, was a surprise last-minute addition to the debate after gaining ground in polls. The big question is whether his rivals and the debate moderators will take him seriously as a threat. The mere fact that Steyer qualified for the debate over such candidates as U.S. Senator Cory Booker suggests he can translate spending into results – something that can keep him in the race long-term. His task will be to frame himself as more than a curiosity and offer an effective response should Sanders or Warren assail him for buying his way onto the stage.