Cedar Rapids (United States) (AFP) - The rush is on for Democratic presidential candidates in Iowa, with a brief break in the impeachment trial of Donald Trump in Washington and the first vote in the party's nominating process a little more than a week away.

Iowans' first-in-the-country vote -- set for February 3 this year -- often plays an outsized role in shaping the presidential field. But in a year of extraordinary flux among Democratic front-runners, predictions are less sure.

A New York Times poll Saturday did show progressive Senator Bernie Sanders pulling into a timely lead in Iowa, with a sizable, seven-point lead over three candidates who sit in a statistical tie: former South Bend, Indiana mayor Pete Buttigieg, former US Vice President Joe Biden and Massachusetts Senator Elizabeth Warren.

But Warren, who has the largest campaign staff in Iowa and has led in polls there, on Saturday received an influential endorsement from a major Iowa newspaper, the Des Moines Register, which called her "the best leader for these times."

Meantime, Biden continues to hold leads in nationwide voter surveys; Buttigieg had led earlier in Iowa; and both Warren and Senator Amy Klobuchar have touted their coveted joint endorsement from the Times.

The impeachment effect

Trump's impeachment trial before the US Senate, meanwhile, has had a wild-card effect.

It has taken Sanders, Warren, Klobuchar and another Democratic candidate, Michael Bennet -- all of them sitting as jurors in the Senate trial -- temporarily off the campaign trail, leaving an opening for Biden, Buttigieg and others to exploit.

But when the trial adjourned at midday Saturday, giving those senators a day and a half to return to the campaign trail before flying back to Washington, they seized the opportunity. 

"The minute we got done with the hearing yesterday," Klobuchar told ABC on Sunday, "I got out to Iowa, did three events; I have three events today." 

On Saturday, she received the endorsement of the biggest newspaper in New Hampshire, which votes on Feb 11, the second stop on the primary schedule. That state's Union Leader said Klobuchar had "an ability to unite rather than divide, and the strength and stamina to go toe-to-toe with" Trump. 

'Zero justice'

In Washington, White House lawyers on Saturday began their defense of Trump at his historic trial, saying he did nothing wrong in his dealings with Ukraine. House prosecutors in the trial had earlier argued that Trump withheld aid to Ukraine to  pressure his Ukrainian counterpart to open an investigation into Biden and his son Hunter.

Senators will have 16 hours in the coming week to ask questions and consider whether to subpoena witnesses.

Klobuchar on Sunday urged them to do so. "Zero witnesses plus zero evidence equals zero justice," she said. "You can't have a trial without witnesses."

The Iowa caucuses, because they launch the primary season, play an influential but hardly definitive role in anointing a nominee.

They are followed this year by New Hampshire, Nevada (Feb 22) and South Carolina (Feb 29). 

Iowa and New Hampshire are small, predominantly white states, not particularly typical of the broader electorate. 

But on March 3 comes "Super Tuesday," when 14 states vote, including two of the most populous, California and Texas.

Democrats formally nominate their nominee only on July 13, at the Democratic National Convention, held this year in the Midwestern city of Milwaukee, Wisconsin.

The Republican National Convention comes 11 days later, in Charlotte, North Carolina.