Four U.S. senators running for president paused their impeachment trial duties Friday to dash to Iowa, joining top rivals for a final weekend of campaigning before the state's all-important vote launches the Democratic Party's nomination contest.

After a Senate vote Friday paved the way for President Donald Trump's acquittal of charges of abuse of power and obstruction of justice, the impeachment trial was put on hold, allowing all candidates to enjoy a breathless final 72 hours of campaigning before the Iowa caucuses.

Polls show a tight contest in the heartland state, where former Vice President Joe Biden this week has been taking his "Soul of the Nation" bus tour to all corners in hopes it will help him cross the finish line in the top spot on Monday.

Former Indiana Mayor Pete Buttigieg, who is running in the same moderate lane as Biden, is also on the ground, arguing it is time to turn the page to more forward-looking, galvanizing leadership.

Biden, 77, "is making the case that this is no time to take a risk on someone new," Buttigieg, who at 38 is less than half Biden's age, told a crowd at the Masonic Temple in the town of Clinton, Iowa.

"I would argue, this is not the time to take the risk of falling back on the familiar or relying on an old playbook that helped get us to this point," Buttigieg said.

The contest to see who challenges Trump in November's election is a month-long slog, and those with minimal support have been peeling off. Former Congressman John Delaney dropped out Friday.

Eleven contenders remain in the race, and several have pitched up seeking to woo undecided Iowa voters, snatch a victory here, and claim the all-important momentum going forward to the next contests, starting with New Hampshire on February 11.

Far-left Senator Bernie Sanders, who has a narrow lead in Iowa over Biden and Buttigieg, progressive Senator Elizabeth Warren, who is in fourth place in polling, and fifth-place Senator Amy Klobuchar were jetting in from Washington to mount their 11th hour pushes for support.

Trump's Senate impeachment trial has effectively tethered them to Washington, but the proceedings adjourned Friday after the chamber's Republican majority rejected Democratic efforts to subpoena witnesses and documents.

Closing arguments will be made Monday, with a final acquittal vote Wednesday.

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'Down to the wire'

For two weeks, the senator candidates have been campaigning with one hand tied behind their back, sending surrogates including relatives and high-profile lawmakers to campaign in their stead.

Among them is Sanders, 78 and the oldest candidate in the race, who enjoys strong support from young voters.

His team organized a campaign concert featuring indie rock band Bon Iver on Friday near Des Moines.

Sanders phoned in from Washington, rallying supporters to "do everything you can" to boost caucus turnout.

The senator is set to host a similar show featuring Vampire Weekend on Saturday in Cedar Rapids.

Warren is scheduled to attend a Friday night Get-out-the-Caucus rally in Des Moines, while Klobuchar is lined up for four events Saturday, at a brewery, a music venue, a women's club and a school.

Nearly half of Democratic Iowa voters said they remained undecided before Monday's caucuses, the local meetings where participants align behind their candidates in one of the country's quirkier voting systems.

Among them is Stephanie Hull, a 21-year-old student who came to see Biden in the small town of Burlington.

"It's a possibility" that President Barack Obama's White House wingman earns her vote, she told AFP.

"I have a few other favorites right now, so it's going to be down to the wire."

Biden, whose global experience is unmatched among the candidates, has declared Trump a danger to America due to his erratic foreign policy.

"The next president of the United States is going to inherit a country that's divided and a world in disarray," Biden told attendees in his half-hour stump speech.

"There's going to be no time for on-the-job training."

Biden's blue-collar roots and his uncanny ability to connect personally with voters are huge assets in a state used to face-to-face encounters with candidates, although his Iowa speeches have lacked the passion that animates some rivals.

Despite the agonizing over which political approach to take – revolution or realism – in the 2020 election, Iowa's Democratic Party chairman Troy Price said voters have unified around one goal.

"There's a lot of people that want to make sure that we defeat Donald Trump," Price said.

Retired teacher Pat Carr, 69, thinks Buttigieg – a millennial gay military veteran – is best-suited to do that.

In an era of hyper-partisanship, he said, "I think Pete's a person who might be able to unite a little better than some of the others."