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 at a Des Moines middle school. "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. "I think he's a communist," Trump told Fox News in an interview that aired before the Super Bowl got underway Sunday.

With campaigns loath to door-knock during American football's championship game, some candidates like Sanders and Klobuchar attended Super Bowl watch parties. Underdog Klobuchar argues that her Midwestern roots and propensity to work with Senate Republicans can help her win Iowa and defeat Trump.

She compared the big game to the last-gasp nature of the final weekend before Iowa's vote. "I would call it the Super Bowl of campaigns," she told supporters in Cedar Rapids.

Turnout will be critical, as candidates seek to persuade voters on issues including health care, improving conditions for the working class and ending Washington corruption. They were also pushing their own electability, as Buttigieg did repeatedly on the stump and during Sunday TV talkshows. "I certainly think that I am better positioned to beat Donald Trump than any of my competitors," Buttigieg told CNN.

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

One in two Iowa voters claimed to still be undecided ahead of the quirky caucus process. Among them was Kim Robinson, 67, a precinct caucus chair in Clive who said he switched support Sunday from Biden to Buttigieg. "And I might change to Amy Klobuchar" before the vote, he told AFP. "Right now it's a matter of who I think will win" against Trump.

At 7:00 pm Monday (0100 GMT Tuesday), Democrats take part in caucuses at about 1,700 venues -- schools, libraries, churches -- to publicly declare their choice by standing under one candidate's banner. Candidates who reach 15 percent support can 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.

At that point, the rallies and TV advertising fade away, as neighbors seek to convince and persuade the undecided, or those from unviable candidates, to align with another. 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.

Ayana Elizabeth Johnson, a marine biologist, traveled from far-off New York to help Warren. "Having a good showing in Iowa is so important to build momentum," Johnson said.