WASHINGTON (AFP) - As the timing of United States President Donald Trump's impeachment trial clashes with the Democratic nomination race, some presidential candidates pinned to Washington are deploying relatives, high-flying supporters - even a dog - as surrogates for critical 11th-hour campaigning in early-voting states.

Of the 100 senators fulfilling their constitutional obligation to sit in judgment of the President, four of them are White House hopefuls.

The ultimate "jury duty" is sidelining two top candidates, Ms Elizabeth Warren and Mr Bernie Sanders, as well as rising second-tier hopeful Amy Klobuchar and lower-performing Michael Bennet, during a proceeding that could last two full weeks.

The trial - where senators are barred from speaking or using electronics during hours of oral arguments - is forcing the quartet to concoct innovative campaign solutions ahead of Iowa's Feb 3 vote, the first in the nomination process.

Mr Sanders has confirmed he is chartering a private plane to jet to and from Iowa for face time with voters, likely during Sunday breaks in the trial.

"I'm not going to be able to be here as much as I would like," he told supporters on Monday at an Iowa museum. "So you guys are going to have to carry the ball."

Ms Klobuchar was due to host a statewide "tele-townhall" with Iowans just half an hour after the trial's Wednesday session wrapped up.

Unable to be in two places at once, the candidates have dispatched their spouses or children to early voting states.

Ms Klobuchar's husband John Bessler and their daughter Abigail, who is in her 20s, will host "Hotdish House Parties" - with dishes like casseroles that are Midwestern staples - as they campaign across Iowa this week, Ms Klobuchar's campaign said.

"As we speak, my daughter is out there," Ms Klobuchar told CNN on Wednesday during a break in the impeachment trial.

"The way politics work, you've got to have people that are out there for you, or you don't have a campaign."

Abigail meanwhile has also taken over her mother's Twitter account while Ms Klobuchar is grounded in the trial.

Ms Warren's husband Bruce Mann, a Harvard professor, campaigned for her in Iowa last week and will attend canvassing events in her stead on Saturday and Sunday in New Hampshire, which votes second in the process, on Feb 11.

On Saturday, Mrs Jane Sanders campaigns for husband Bernie in Nevada, the western state that votes one month from Wednesday.

'FABULOUS' SURROGATE STORY

Ms Jennifer Rosenbaum, who headed Mr Barack Obama's surrogate operation during his 2012 presidential re-election, said it is "unfortunate" that the four senators cannot campaign during such an inflection point for the country.

But she does not believe their campaigns will suffer due to their trial commitment.

"They're in Washington, so who's actually covering for them? I think it's a fabulous story for all four," she told AFP.

Voters long to see the human nature of candidates, "and they're glad to see that there are family members out stumping", she added. "And don't forget about the four-legged creatures."

There is no official word whether Ms Warren's husband will be accompanied in New Hampshire by Bailey, their golden retriever who has become an unofficial mascot - a "furrogate", perhaps - and has campaigned with Ms Warren before.

Mr Sanders' rally on Wednesday at the University of Northern Iowa was cancelled "due to the impeachment schedule" in Washington.

But popular liberal congresswoman Alexandria Ocasio-Cortez, who has endorsed Ms Sanders and can draw substantial crowds herself, is set to campaign for him in Iowa on Friday and Saturday.

Ms Warren, too, has heavy hitters campaigning for her, including Mr Julian Castro - who endorsed her shortly after dropping out of the race this month - and several lawmakers.

While relatives stand in for their loved ones as candidates trade the frigid Iowa cornfields for the Senate floor's more temperate climate, front runner Joe Biden and fellow moderate Democrat Pete Buttigieg are barnstorming Iowa unencumbered.

Mr Buttigieg, the former mayor of South Bend, Indiana, held four town halls on Tuesday in the Hawkeye State while senators endured a marathon 13-hour impeachment trial session.

But Mr Biden and Mr Buttigieg may not have a massive advantage over their rivals, argued Ms Rosenbaum.

"Really good surrogates can act as great messengers for a candidate, especially leading into a primary," she said.