With the first test of the United States presidential election less than a week away, Democratic hopefuls fanned out across Iowa over the weekend hoping a last-ditch bout of rallies and stump speeches will pay off when Iowans huddle for their biennial caucuses on February 3.

The number of candidates to choose from has narrowed from more than 25 earlier this year to a field of 12, but little else has changed in the Democratic dynamic over the last year. Who will walk away with the coveted top spot is as unclear today as it was a year ago.

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Polls released over the weekend only muddied the waters. One had Vermont Senator Bernie Sanders with a seven-point lead. Another had former Vice President Joe Biden up by six points. Yet another was showing a dead heat between Sanders, Biden and former South Bend, Indiana Mayor Pete Buttigieg. Massachusetts Senator Elizabeth Warren was in fourth place in all the polls.

"There's still plenty of time for movement," Kurt Meyer, a Democratic official in northern Iowa, told The Associated Press news agency. "Every part of the ground game counts."

Democratic 2020 US presidential candidate and US Senator Bernie Sanders speaks at a campaign rally in Sioux City, Iowa [Ivan Alvarado/Reuters]

Complicating that ground game for candidates Warren, Sanders, Amy Klobuchar, and Michael Bennet is the fact that, after just one day in Iowa, the US senators were forced back to Washington, DC, to hear more of the impeachment trial against President Donald Trump. As sitting senators, all four are constitutionally obligated to be present during the proceedings and will have to rely heavily on surrogates during the final week of campaigning in Iowa.

Biden, free from such constraints, kicked off a statewide bus tour - the "Soul of the Nation" - on Sunday with scheduled stops in 20 cities in 17 counties before heading back in Des Moines on February 2. Currently leading most national polls, Biden has avoided targeting his fellow Democrats and focused instead on the November vote.

"I don't believe we are the dark, angry nation that Donald Trump tweets about at night," Biden told a crowd just outside Des Moines Sunday. "We are so much better than Donald Trump."

The Buttigieg campaign, for its part, announced a series of events in so-called "pivot counties" that voted for Barack Obama in 2012 but switched to Trump in 2016. Iowa has more such counties than any other state in the country, and the effort is intended to show that the political newcomer can siphon off disaffected Republicans from the president in November.

Democratic presidential candidate and former South Bend, Indiana mayor Pete Buttigieg and his husband Chasten Buttigieg attend a campaign event in Boone, Iowa [Eric Thayer/Reuters]

In an advertisement released on Sunday, billed as his last before the Iowa vote, the 38-year-old Buttigieg tries to turn his status as the youngest of the candidates to his advantage.

"It's time to turn the page from a Washington experience paralysed by the same old thinking, polarized by the same old fights, to a bold vision for the next generation," Buttigieg says in the advertisement. "We need to break from the old politics and unify this nation."

Even President Trump, who as incumbent faces no realistic challenge in Iowa's Republican caucuses, was eager to get in on the action, dispatching dozens of family members, cabinet officials and other GOP dignitaries to the state to try and capture some of the limelight.

Democratic 2020 US presidential candidate and former Vice President Joe Biden speaks during a campaign event in Cedar Falls, Iowa [Carlos Barria/Reuters]

The other major Democratic candidate, former New York City Michael Bloomberg, has taken the unconventional approach of largely ignoring Iowa in favour of bigger states such as Florida and Texas that vote in early March. In a bid for the sizable Puerto Rican vote in Florida, Bloomberg said Monday that he would support a bid by the island nation to become the 51st American state.

The first-in-the-nation exercises in Iowa, while not direct votes for individual candidates, can make or break campaigns in such a crowded field with no clear frontrunner. Since they began in 1972, the first-place winner has gone on to secure the nomination in seven of the last 10 primary contests, including the last four.

Democratic officials in the state have said they are expecting a heavy turnout next week, largely because of the organising efforts of the candidates and the polarising nature of Trump's presidency. State Chairman Tony Price told Politico that the party is expecting even more than the 239,000 caucus-goers who turned out in 2008, a record.

Democratic 2020 US presidential candidate and US Senator Elizabeth Warren reacts to the crowd after speaking at a campaign town hall meeting in Cedar Rapids, Iowa [Brian Snyder/Reuters]

A win in Iowa for either Sanders or Warren would solidify the candidate's status as the torch-bearer for the more progressive wing of the party. But first place is not the only prize. For centrists such as Buttigieg and Klobuchar vying for the same voters as Biden, merely showing up in the top three would give them a boost.

Already last week, the Warren camp seemed to sense the hurdles ahead of it in Iowa and began downplaying the results of the next week's caucuses. In a memo released on Friday and obtained by The Hill newspaper, Warren campaign manager reminded supporters "the four early states contests are just the beginning", and that the Massachusetts senator was in the race for the long haul.