Democratic presidential candidate former Vice President Joe Biden speaks at a town hall event in Des Moines, Iowa, on Sunday. Democratic candidates raced across Iowa on Sunday in a last-minute flurry of rallies and hand-shakes ahead of the state's nominating vote that marks the official start of the US presidential election season. Iowa has traditionally served as a vital launching point - or burial ground - for presidential hopefuls. Photo: AFP

 
Democratic candidates made their frantic, final campaign pitches Sunday in Iowa, on a mission to persuade undecided voters one day before the state's nominating contest officially starts the US presidential election season.

Iowa, a largely rural state of three million people, has traditionally served as a vital launching point - or burial ground - for presidential hopefuls.

Even as all eyes turn to the debut vote, Donald Trump's US Senate impeachment trial weighs over the Democratic kick-off, with the president expected to be acquitted just days after the Iowa contest.

"This is the most consequential election, certainly in the modern history of this country ... and it all begins tomorrow night," Senator Bernie Sanders, the leading progressive in the race, told invigorated supporters at a meet-and-greet event in Iowa City.

Similar scenes played out across the state this weekend as most of the 11 remaining candidates made their final push to convince undecided voters that they are best positioned to defeat Trump.

Monday's caucuses have created an air of suspense with no clear frontrunner. Several hopefuls look to strike gold here and seize the momentum going into the next contest, in New Hampshire on February 11.

Leftist Sanders holds only a narrow lead over moderate former vice president Joe Biden. South Bend, Indiana ex-mayor Pete Buttigieg and Senator Elizabeth Warren are mere points behind.

"Tomorrow, you can ruin Donald Trump's night!" Biden said as he rallied 1,100 supporters in Des Moines. "I promise you: If you stand with me, we will end Trump's reign of hatred and division."

Three of the leading candidates seized on a brief break from their duties as impeachment jurors to barnstorm Iowa.

The senators - self-styled democratic socialist Sanders, progressive Warren and pragmatist Amy Klobuchar - each hosted multiple events Sunday.

The impeachment trial - only the third in history of a US president - created an unprecedented situation by limiting the senators' ability to campaign ahead of Iowa's vote.  

They must return to Washington Monday for the trial's resumption.

Senate leaders have scheduled a Wednesday vote that will almost certainly acquit Trump on charges of abuse of power and obstruction of Congress.

Trump will likely claim victory over Democrats' efforts to oust him when he delivers his State of the Union speech Tuesday - although he told reporters he would be delivering "a very, very positive message." That did not stop him from branding Biden "Sleepy Joe" and hurling an epithet at Sanders.

Iowans take their role as first-in-the-nation voters to heart, and their pick has a recent historical track record of becoming the Democratic nominee.

At an Iowa City home that Warren supporters were using as a base, people streamed in and out, looking for extra posters or lists of doors that still needed to be knocked on.