Six US Democratic presidential hopefuls will take the stage at Drake University in Des Moines, Iowa on Tuesday night for the seventh of 10 planned debates.

The debate, hosted by CNN and the Des Moines Register newspaper, is set to begin at 8pm local time (02:00 GMT on Wednesday).

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It is the last before the all-important Iowa caucuses on February 3, making it arguably the most important night of the primary season so far. The latest polls from the state, most notably a Des Moines Register/CNN/Mediacom released on Friday, shocked many observers by showing Senator Bernie Sanders in the lead for the first time. Some 20 percent of likely Democratic caucus-goers said Sanders was their first choice in the race. A subsequent poll released on Monday, however, had Biden ahead of Sanders by six points.

The voting in Iowa, the first of the 2020 election year, plays an outsized role in the selection of nominees for president. Every Democrat in history who won both Iowa and New Hampshire, which holds the nation's first primary election (as opposed to a caucus) on February 11, went on to become the nominee. A win in Iowa would give Sanders a big boost as he moves on to New Hampshire.

As the six Democrats prepare to take the stage, here's all you need to know:

1. Which candidates will be on stage? What threshold did they have to meet?

Six of the 12 Democratic candidates still vying for the nomination will be on stage in Des Moines, with Sanders and former Vice President Joe Biden at its centre. Other participants include billionaire Tom Steyer, Senator Elizabeth Warren, former South Bend, Indiana, Mayor Pete Buttigieg and Senator Amy Klobuchar.

The number of participants has narrowed considerably since the debates began in June, when they had to be spread out over two nights to accommodate all the candidates. Some in the progressive wings of the party believe it has narrowed too much - Tuesday's debate will be the first all-white debate because New Jersey Senator Cory Booker, businessman Andrew Yang, and former Massachusetts Governor Deval Patrick failed to meet the threshold for participation. On Monday, Booker announced that he was ending his campaign.

To qualify, the candidates needed to receive at least 5 percent support in at least four Democratic National Committee-approved national or early-voting-state (Iowa, New Hampshire, South Carolina, Nevada) polls, or 7 percent in two early state polls. The candidates also required donations from at least 225,000 individual donors and a minimum of 1,000 unique donors per state in at least 20 states.

2. Which candidates will not be on stage?

In addition to Patrick and Yang, US Senator Michael Bennet, former US Congressman John Delaney, US Congresswoman Tulsi Gabbard and former New York City Mayor Michael Bloomberg failed to qualify for the January debate. Bloomberg, a late addition to the race, is not accepting donations to his unconventional campaign. He has opted to eschew retail campaigning in early states such as Iowa and New Hampshire and instead spend hundreds of millions on advertising with an eye towards coming out ahead on Super Tuesday in March, when 13 states all hold primaries on the same day.

3. What issues will likely come up?

Iran

The debate comes less than two weeks after President Donald Trump ordered the US killing of top Iranian military commander Qassem Soliemani in Iraq. That drone strike prompted Iran to attack two Iraqi bases housing US troops a week ago. There were no casualties.

Trump has since been criticised for his stated justification for ordering Soleimani's killing. The Trump administration maintains Soliemani was planning "imminent attacks" against US forces, but conflicting information and a lack of publicly released intelligence has Democrats and some Republicans demanding more answers.

Expect US foreign policy in the Middle East, especially in regards to Iran, to come up on Tuesday night. The candidates will likely hammer Trump for his handling of the escalating tensions with Iran and parrot recent polls showing that most Americans feel the world is less safe because of the US president's actions.

Iraq

Iraq is likely to be a headache for Biden. His challengers have criticised the former senator for supporting military action in Iraq following the September 11, 2011, attacks in the United States. Sanders and Buttigieg have both stepped up their attacks on Biden on the issue, the latter calling the vote one of the "worst foreign policy decisions" of his lifetime. Expect Biden to try and fend off the criticism by championing his eight years of foreign policy experience while vice president.

Impeachment

With a House vote expected on Wednesday to send the articles of impeachment to the Senate for a full trial, the six Democrats will probably not be able to avoid the topic. Whether they linger on it or not remains to be seen. National polls suggest that the topic is not changing any voters' minds about the president.

Healthcare

The bread-and-butter issue of healthcare in America is almost certain to come up. Warren has backed off her support for a government-run healthcare system slightly since taking some heat on the topic earlier, but Sanders's recent surge in the polls may have her rethinking that retreat. Their more moderate challengers are likely to continue calling the progressive plans for Medicare-for-all unworkable and political suicide in the general election.

Sanders v Warren

The progressive wing of the Democratic party, personified by Warren and Sanders, has so far avoided open squabbling, but that changed this week when the Sanders campaign suggested that Warren is an elitist candidate and someone leaked details of a private conversation between the two candidates in which Sanders is alleged to have said a woman can't win the nomination.

Sanders initially denied making the remark, and Warren's camp at first declined to comment. But following multiple media reports, Warren said in a statement on Monday night that among the topics the pair discussed in a two-hour meeting in December 2018 "was what would happen if Democrats nominated a female candidate".

Warren said she thought a woman could win. "He disagreed," she said in the statement. "I have no interest in discussing this private meeting any further because Bernie and I have far more in common than our differences on punditry," she added.

More fireworks between the two tonight could come up.

4. What about Trump?

Trump will head to the critical battleground state of Wisconsin for a campaign rally on Tuesday night.

Trump's event in Milwaukee is scheduled to kick off one hour before the Democratic debate in Des Moines.

During previous debates, the president often called the Democratic candidates "boring" and continued to criticise the media.

5. When is the next debate?

The eighth Democratic debate is scheduled for February 7, just four days before the New Hampshire primary, in - fittingly - Manchester, New Hampshire.