Supporters of Democratic candidate Elizabeth Warren try to sway an undecided voter during caucusing at Abraham Lincoln High School in Des Moines, Iowa, on Monday. — AFP/VNA Photo

DES MOINES, United States — Americans on Monday kicked off the first vote of the 2020 presidential race as Iowa opened its caucuses, the closely-watched first step in deciding which Democrat faces incumbent Donald Trump in November.

The two frontrunners, far-left senator Bernie Sanders and moderate former vice president Joe Biden, face a key test in the sparsely populated state, with a handful of others looking to make their mark and earn momentum going forward.

In a unique process held across nearly 1,700 sites – schools, libraries, churches and meeting halls – the Iowa vote offers a critical early look at the viability of the 11 Democrats still in the race – even though just 41 delegates are up for grabs, a fraction of the 1,991 needed to secure the nomination in July.

Luke Elzinga, a volunteer for Sanders, appeared early at Lincoln High School in Des Moines which was converted into a caucus location.

"I think he really inspires a lot of young people, a lot of disaffected voters who might not otherwise turn out," said Elzinga, 28.

"And so I think he's the best candidate to beat Trump."

Sanders and Biden lead in Iowa, followed by former South Bend, Indiana mayor Pete Buttigieg and progressive senator Elizabeth Warren. But polling has fluctuated and Iowa's quirky caucus system – where voting is not by secret ballot but by public declaration for a candidate – can make the night hard to predict.

Caucusing at the same Des Moines location, Rosalie Gallagher – the 74-year-old owner of an interior design business – came to support Biden, and to bar the road to the likes of Sanders and Warren.

"We are never going to win with a leftist candidate!" she said. "If they win, the Democrats will lose. Period."

Sanders ahead

As the White House nomination race kicked off in Iowa, another hugely consequential political process was nearing its conclusion: the Senate impeachment trial of Trump, the man Democrats hope to see off in the election exactly nine months away.

Trump is almost certain to be acquitted Wednesday by the Republican-led chamber on charges of abuse of power and obstruction of Congress.

Three Democratic candidates – Sanders, Warren and moderate Amy Klobuchar – have faced the unprecedented scenario of spending much of the past two weeks tethered to Washington for the impeachment trial instead of on the campaign trail.

Even at the 11th hour, the senators were obligated to return to Washington for the trial's closing arguments on Monday.

Second-tier hopefuls Klobuchar and tech entrepreneur Andrew Yang look meanwhile hope to outpace expectations and seize momentum heading into the next contest, in New Hampshire on February 11.

Earlier on Monday Biden – who still holds the lead in national polls – brought pizza to a field office in a strip mall near Des Moines to thank volunteers.

"I'm feeling good about today," he said, before donning sunglasses indoors and addressed volunteers.

Like many candidates, Biden spent the weekend crisscrossing Iowa in a final push to convince undecided voters he is best placed to accomplish Democrats' number one goal: defeating Trump.

The president has not stood idly by. On Sunday, he branded Biden "Sleepy Joe" and described Sanders as "a communist", previewing a likely line of attack were Sanders to win the nomination.

Trump is all but certain to be confirmed as the Republican nominee, but his two sons and other surrogates were nevertheless in Iowa Monday making his case to Republicans who are also holding caucuses.

Surprises in store?

Unlike secret ballot voting, caucus-goers publicly declare their presidential choice by standing together with other supporters of a candidate.

Candidates who reach 15 per cent support earn delegates for the nomination race while supporters of candidates who fall short can shift to others.

Turnout is critical, and candidates and their representatives will seek to persuade voters on issues including health care, taxes and ending Washington corruption.

One key candidate who has opted not to contest Iowa is billionaire businessman Michael Bloomberg, who entered the race in November but has surged into fourth place in RealClearPolitics' national polling average.

The former New York mayor, who has spent more than $300 million on advertising, according to Advertising Analytics, is focused on running a national campaign with particular emphasis on states – such as California – that vote on "Super Tuesday", March 3.

"Today there's something going on in Iowa I heard, something like that," Bloomberg said at an event in Compton, California on Monday.

"California's population and economy are the largest in the nation," he said. "I am here and I am here to win." — AFP