Months into the Democratic nomination marathon, after seven debates, countless rallies, bitter candidate clashes and an impeachment effort to remove the US president, Iowa on Monday holds the first-in-the-nation vote to see who challenges Donald Trump in November.

More than two dozen White House hopefuls began the journey, some as early as a year ago. Eleven now remain, exactly nine months from Election Day.

Despite the historically diverse field consisting of men and women of colour and young candidates with little Washington exposure, the two frontrunners are septuagenarian white men -- Bernie Sanders and Joe Biden -- with more than 80 years of political experience between them.

The heartland state's all-important vote will likely whittle the field further as it provides the first verifiable results in a contest deciding the party's future direction, and its 2020 flagbearer.

In a typical election year, Iowa absorbs the country's full political attention. But this presidential cycle has been anything but normal.

Looming over the process is Trump's impeachment saga, which is coming to a climax on the same week Iowa votes.

Three main senator candidates -- progressives Sanders and Elizabeth 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, essentially leaving them to campaign in Iowa with one hand tied behind their back.

A final vote on Trump's fate, with acquittal almost certain in the Republican-led Senate, occurs Wednesday.

Add January's military scare with Iran, a stubborn trade war with China, a deadly outbreak of novel coronavirus that has set parts of the world on edge -- and US Democratic hopefuls find themselves competing for headlines.

- Sanders ahead -

Sanders, running as a democratic socialist, is leading in Iowa. An Emerson College poll of Democratic voters released on the eve of the caucus shows him with 28 percent support, seven points clear of centrist Biden, the former vice president who is the national frontrunner.

South Bend, Indiana ex-mayor Pete Buttigieg and Warren are about four points behind Biden in poll averages.

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

"This is the most consequential election, certainly in the modern history of this country... and it all begins tomorrow night," Sanders told supporters Sunday in Iowa City.

Similar scenes played out across the state this weekend as candidates made their final frantic push to convince undecided voters that they are best positioned to send the controversial incumbent packing.

"I promise you: if you stand with me, we will end Trump's reign of hatred and division," Biden said as he rallied 1,100 supporters at a Des Moines middle school.

Trump has not stood idly by. The pugnacious president has repeatedly attacked Democrats, and did so Sunday, branding Biden "Sleepy Joe" -- his campaign events often lack the pizzazz of rivals -- and hurling an epithet at Sanders.

"I think he's a communist," Trump told Fox News, previewing a likely line of attack were Sanders to win the nomination.

- Surprises in store? -

Iowa's stage is set. At 7:00 pm (0100 GMT Tuesday), nearly 1,700 schools, libraries, churches and other venues welcome the state's registered Democrats to participate in a quirky, sometimes chaotic American ritual.

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

I certainly think that I am better positioned to beat Donald Trump than any of my competitors - Buttigieg

Candidates who reach 15 percent support earn delegates for the nomination race. If a candidate does not meet this threshold after the first alignment of caucus-goers, their supporters can shift to other candidates, a process that potentially can reorder the rankings.

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

They are also pushing their own electability, as Buttigieg did repeatedly Sunday on the stump and during TV talk shows.

"I certainly think that I am better positioned to beat Donald Trump than any of my competitors," Buttigieg told CNN.

A former US Navy reservist who became a mayor at 29, Buttigieg portrays his youth and new ideas as reasons voters should prefer him over the gray-haired Biden, 77, and Sanders, 78.

The caucuses could yield major surprises, as one in two Iowa voters claimed still to be undecided.