DES MOINES, IOWA - Six Democrats braced for Tuesday's final presidential debate before Iowa kicks off the nomination process February 3, and with no candidate yet gaining the upper hand, they were ready to rumble.

Former vice president Joe Biden is leading nationally, but no clear frontrunner has emerged in Iowa, and candidates are scrambling for a breakout moment that could translate into the first tangible victory in a tight contest over who challenges President Donald Trump in November.

With the battle intensifying, some of the sharpest exchanges of the year-long campaign were leveled between top liberals Bernie Sanders and Elizabeth Warren, senators and self-described friends whose cordiality accord appears to have unraveled.

The debate in Des Moines, Iowa risks being overshadowed by another historic political drama: Trump's looming impeachment trial in the US Senate, which -- if it begins later this week or next -- would force four presidential candidates to stay in Washington serving as jurors in the proceedings.

Tuesday's nationally televised showdown will feature the smallest stage yet -- and its whitest, with no people of color qualifying for the debate for the first time in the 2020 race.

The Democratic Party last year hailed its historically diverse presidential field, but one by one minority candidates have fallen by the wayside, the latest being African-American Senator Cory Booker, who dropped out on Monday.

The debate features establishment candidate Biden, progressives Sanders and Warren, centrists Senator Amy Klobuchar and former South Bend mayor Pete Buttigieg, and billionaire activist Tom Steyer.

As Iowa nears, Tuesday may spell the most consequential moment of the campaign to date.

Fortified by his resilient showing in recent polls despite suffering a heart attack last November, Sanders appeared eager to seize the moment and went on the offensive against rivals.

He savaged Biden with a pointed statement criticizing the ex-senator's vote in support of going to war in Iraq, a subject that has gained renewed review with Trump's ordered killing of an Iranian commander in Baghdad and the subsequent fallout.

"It is appalling that after 18 years Joe Biden still refuses to admit he was dead wrong on the Iraq War, the worst foreign policy blunder in modern American history," Sanders senior advisor Jeff Weaver said in a statement.

Sanders has also apparently chided Warren as the candidate of the elite, with Politico reporting that Sanders volunteers were instructed to tell voters that Warren could not expand the Democratic base in a general election.

Warren said she was "disappointed" by the report that Sanders sent volunteers to "trash" her, telling reporters in Iowa that "I hope Bernie reconsiders and turns his campaign in a different direction." -AFP