Six US Democratic presidential hopefuls took the stage on Tuesday in the last debate before Iowa voters kick off the Democratic nominating race on February 3.

The debate saw clashes over foreign policy, trade and other issues.

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On stage in Des Moines, Iowa, were US Senators Bernie Sanders, Elizabeth Warren and Amy Klobuchar, former Vice President Joe Biden, South Bend, Indiana, Mayor Pete Buttigieg and businessman Tom Steyer.

Six other candidates failed to qualify for the debate.

For those on stage, the debate offered the candidates a final high-stakes chance to make their case to voters.

Here's what was said on some of the top issues:

Foreign Policy

The evening began with the candidates clashing over Iraq, war and foreign policy, although they were largely united against President Donald Trump's leadership on such issues.

Sanders, a longtime antiwar advocate who voted against the 2002 authorisation of war in Iraq, criticised Biden for supporting the war and said they heard the same arguments from officials in former President George W Bush's administration before coming to different conclusions.

"I thought they were lying, I did not believe them for a moment," Sanders said. "I did everything I could to prevent that war. Joe saw things differently."

Democratic 2020 US presidential candidate and former Vice President Joe Biden smiles as Senator Bernie Sanders speaks during the seventh Democratic 2020 presidential debate at Drake University in Des Moines, Iowa [Shannon Stapleton/Reuters]

Biden, a former chairman of the Senate Foreign Relations Committee who touts his security credentials, acknowledged the vote "was a big, big mistake" and said that as President Barack Obama's vice president, he worked to bring the troops home.

"It was a mistake to trust that they weren't going to go to war," Biden said of the Bush administration. "It was a mistaken vote, but I think my record overall on everything we have done, I'm prepared to compare it to anybody's on this stage."

Several candidates condemned Trump's recent decision to order the killing of Iran's top general, Qassem Soleimani, and his decision to keep US troops in the region.

"We have to get combat troops out," declared Warren, who also called for reducing the military budget.

Democratic 2020 US presidential candidates former Vice President Joe Biden, Senator Bernie Sanders, former South Bend Mayor Pete Buttigieg and Senator Amy Klobuchar participate in the seventh Democratic 2020 presidential debate in Des Moines, Iowa [Shannon Stapleton/Reuters]

Others, including Buttigieg, Biden and Klobuchar, said they favoured maintaining a small military presence in the Middle East.

"I bring a different perspective," said Buttigieg, who was a military intelligence officer in Afghanistan. "We can continue to remain engaged without having an endless commitment to ground troops."

'Fundamental difference' on trade

The night also saw clashes on trade.

Sanders said he and Biden had "a fundamental difference" on the worth of regional free trade agreements like Trump's new agreement with Mexico and Canada, called the USMCA, which Sanders opposes and Biden backs.

"I don't know that there's any trade agreement that the senator would ever think made any sense," Biden said.

Sanders said the agreements "were written for one reason alone, and that is to increase the profits of large multinational corporations."

Klobuchar and Warren said they planned to vote in favour of the USMCA.

A female president

Warren made a vigorous case for a female president following a pre-debate feud with progressive rival Sanders.

"Look at the men on this stage. Collectively they have lost 10 elections," Warren exclaimed, "The only people on this stage who have won every single election that they've been in are the women."

Sanders vehemently denied Warren's accusation that he said in 2018 a woman could not beat Trump.

"Does anybody in their right mind think a woman can't be elected president?" Sanders asked. "Of course a woman can win."

He added: "I don't know that that's the major issue of the day."

Democratic 2020 US presidential candidate Senator Elizabeth Warren speaks as and former Vice President Joe Biden and Senator Bernie Sanders listen during the seventh Democratic 2020 presidential debate in Des Moines, Iowa [Shannon Stapleton/Reuters]

The feud came as Democrats try to navigate broader debates over how to reflect and embrace the crucial role female and minority voters will play in 2020.

For the first time, not a single candidate of colour appeared on stage. All six candidates who met the party's polling and donor thresholds were white, and four were men. To defeat Trump at the end of this year, Democrats need to ensure black, Latino and suburban voters are excited to vote against the Republican president.

"The biggest mistake we can make is take black votes for granted. I never will," Buttigieg said when questioned about his lack of support among African Americans.

Candidates also discussed climate change, healthcare and what kind of candidate they would be against Trump.