In the wake of escalating US-Iran tensions, foreign policy is expected to be a key issue during tonight's debate.

Here's what frontrunners Joe Biden, Elizabeth Warren and Bernie Sanders had to say about the air strike on Iranian general Qasem Soleimani while on the campaign trail earlier this month.

• The billionaire candidate has made climate change his signature campaign issue, but has faced accusations that he's trying to buy the presidency
• The 62-year-old former hedge fund manager regularly decries "the corporate stranglehold on democracy"
• Steyer has no prior experience in office, but has already directed tens of millions of dollars into TV campaign commercials
• His campaign slogan is "Fixing a Broken Washington"

• The 37-year-old Afghan war veteran served as the mayor of South Bend, Indiana - a city with a population of just over 100,000 residents - from 2012 until this month
• He was educated at Harvard and Oxford and later went to work for management consultancy McKinsey
• If elected, he would be the youngest and first openly gay American president

President Trump, speaking at his campaign rally in Wisconsin, wasted no time swiping at his Democratic rivals.

"Crazy Bernie [Sanders], he's surging!" Trump said, to a chorus of boos from the crowd.

After a brief pause for some protesters to be escorted out, including one with a Bernie sign, Trump took on Sander's Democratic rival Elizabeth Warren.

"Elizabeth Pocahontas Warren," he said, using his nickname for the Massachusetts senator over her purported Native American heritage.

"She said that Bernie said that a woman cant be president. I don't believe that, I really don't believe that."

This week, Warren claimed her fellow progressive candidate had said in 2018 that a woman could not win this year's presidential election. Sanders denied making the remark.

• Are voters biased against women candidates?
• How Trump talks about women - and does it matter?

Here's what President Trump has said about the Democrats getting ready to debate tonight.

On Joe Biden: "Welcome to the race, Sleepy Joe. I only hope you have the intelligence, long in doubt, to wage a successful primary campaign."

On Bernie Sanders: "Bernie is crazy, but Bernie has got a lot more energy than Biden, so you never know."

On Elizabeth Warren: "I think Pocahontas, she's finished. She's out. She's gone. When it was found that I had more Indian blood in me than she did, and then it was determined I had none, but I still had more, that was the end of her 32-year scam."

On Pete Buttigieg: "Alfred E Neuman [Mad magazine cartoon] cannot become president of the United States."

On Amy Klobuchar: "Amy Klobuchar announced that she is running for President, talking proudly of fighting global warming while standing in a virtual blizzard of snow, ice and freezing temperatures. Bad timing. By the end of her speech she looked like a Snowman(woman)!"

On Tom Steyer: "Weirdo."

Read more about all the candidates, their key issues - and what Trump has said - here.

• The 59-year-old senator from Minnesota is running as a moderate who prides herself on being able to "disagree without being disagreeable" with her Republicans counterparts
• She easily won re-election last November in her home state, which Trump nearly won in 2016
• She'll be looking to capitalise on the momentum she gathered in the previous debate, in which she said: "I did not come here to listen to this argument... I came her to make a case for progress"
• Her campaign slogan is: "Let's get to work"

• The 78-year-old self-identified Democratic Socialist attracted a surprising amount of support in 2016 during his primary race against Hillary Clinton
• Although not technically a Democrat, the Vermont independent senator caucuses with Democrats in Congress
• Sanders voted against the 2003 US invasion of Iraq, and has repeatedly criticised rival Joe Biden for his vote in favour of the Iraq war resolution
• His campaign slogan is "Not me. Us."

• The 70-year-old Massachusetts progressive senator previously worked as a financial watchdog for the Obama administration in the aftermath of the 2008 financial crisis
• She was born in Oklahoma and was a registered Republican until 1996
• Warren, a former professor specialising in bankruptcy law, has been called the candidate with a plan for everything, especially overhauling the US financial sector

Tonight's debate is in Iowa - but why is this midwestern state so key to the election process?

Iowa is the first state to hold caucuses - essentially a party-sponsored voting session - to determine which Republican and Democratic candidates will receive the state's delegates.

These delegates will then cast their votes at each party's convention in the summer, where the final presidential nominee is selected.

Iowa is not necessarily a good predictor of the general election, but winning the first state to vote can help candidates gain momentum.

The last four eventual Democratic nominees all won Iowa. As for Republicans, Iowa has only picked the eventual nominee twice since 1980.

But there has been criticism over Iowa's lead position in election season, as the state is among the least diverse in the nation. US Census data shows Iowa is around 90% white, while the national average is closer to 76%.

According to the Pew Research Center, states like New Jersey and Illinois are the most similar to the overall composition of the nation in terms of race, immigration age and urban/rural mix.

Trump has just walked to the podium in Milwaukee for a campaign rally.

He's beginning by flattering the local football team, the Green Bay Packers, and touting his accomplishments in office.

Meanwhile, his campaign is flying a sign over the debate venue in Iowa saying that Trump "fights for Iowa farmers".

Image source, Getty Images

Anthony Zurcher
BBC North America reporter

Image source, Getty Images

In three weeks, Iowa Democrats will begin the party's process for selecting a nominee to take on Donald Trump in the general election.

And tonight they host a high-stakes debate. After more than a year of campaigning, it's almost time for actual voters to begin weighing in - and the race is still wide open.

Lil and Jarad Bernstein stood at the edge of the crowd gathered near the stage after a Pete Buttigieg rally in Des Moines on Sunday.

They were two of the several hundred who had turned out on a snowy evening to hear the former South Bend, Indiana mayor speak, but unlike many of the attendees - who came sporting campaign T-shirts, buttons and hats - they were not entirely sold on him yet.

"I like all of the candidates," Lil said. "I'm really just looking for someone who can win the nomination and ultimately beat Trump in the election."

Read Anthony's full analysis here.

Debates, primaries, caucuses - the process of picking the next US president is lengthy and chock full of new vocabulary.

We've got you covered here.

Image source, Getty Images

President Donald Trump is having a rally tonight in Wisconsin, from where the BBC's Holly Honderich reports.

"This is the best," Renee Mielke says. "I even took off my job… I just wanted to see Mr Donald Trump speak."

I asked if any Democrat looking to face-off again the president this year scared her.

"None of them do," Mielke said.

Trump has her vote - but of all the Democrats squaring off at the debate tonight, Mielke says, Bernie Sanders is her favourite.

"He too is just trying to stand up for what he believes, he doesn’t back down."

Of the 12 Democrats left in the running, only three are non-white - Andrew Yang, Tulsi Gabbard and Deval Patrick.

And, for the first time since the series of debates began last year, you won't see any of them on stage.

Critics have pointed out how similar the candidates who qualified are, given that the diversity of the field was praised at the start of the election season.

Senator Cory Booker, who is African-American, dropped out of the race yesterday, while Julian Castro, the only Latino in the field, also ended his presidential bid this month.

Booker had told MSNBC in December the party was "spiralling towards a debate stage...with no diversity whatsoever".

And this month, the Washington Post ran a column titled "Democrats are starting to look like a ‘Whites only’ party".

Even some Republicans have weighed in:

As the BBC's Anthony Zurcher noted with Booker's departure, the growing lack of diversity is "a challenge the party will have to confront in the days ahead".

Image source, Getty Images

Welcome to the BBC’s live coverage of the seventh Democratic debate, which is being held in Des Moines - the state capital of Iowa.

The debate is the last before Iowa voters go to the polls on 3 February to vote for a party nominee.

The eventual Democratic nominee will be announced at the party convention in July.

The six candidates who qualified for tonight’s debate are Joe Biden, Pete Buttigieg, Amy Klobuchar, Bernie Sanders, Elizabeth Warren and Tom Steyer. All six are white - notable given the initial diversity of the Democratic 2020 field.

Watch for any clashes between progressive senators Bernie Sanders and Elizabeth Warren, who had maintained an informal truce but drew blood in recent days after a story about a private meeting between the pair.

The story, in Politico, said Sanders told Warren in the meeting that a woman could not win the presidency. He categorically denied it. She said it happened. Tonight might see the first on-stage clash between the two.