Four Democrats running to challenge United States President Donald Trump in November's general election have a new headache to deal with - a mandatory obligation that will keep them off the campaign trail for at least two weeks, if not longer.

As sitting US senators, Bernie Sanders, Elizabeth Warren, Amy Klobuchar and Michael Bennet are constitutionally obligated to be in Washington, DC, for the duration of Trump's impeachment trial, which is expected to begin in earnest next week.

That means sitting in silence in the US capital while the others in the race, including frontrunners former Vice President Joe Biden and former South Bend, Indiana, Mayor Pete Buttigieg, spend the last few weeks before critical caucuses and primaries in Iowa and New Hampshire out actually campaigning.

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Polls in Iowa put Biden, Sanders, Warren and Buttigieg within five points of each other - essentially a dead heat. The picture in New Hampshire is not that different, with Biden leading Sanders by less than two points.

Most of the candidates have been cautious about criticising the timing of the impeachment trial, insisting that justice is more important than politics. Reports suggest, however, that some are grumbling privately about House Speaker Nancy Pelosi's decision to withhold the articles of impeachment, passed by the House in December, from the Senate until this week.

Before he dropped out of the race earlier this week, New Jersey Senator Cory Booker was the most open among the Democrats about how much the impeachment timing would have hurt him. A "big, big blow" is how he described it to the Associated Press earlier this month.

"If this trial lasts two weeks, that is literally dozens of events we won't be able to do," Booker said at the time.

The trial in the Senate is to fully get under way as early as Tuesday and to continue six days a week - Sundays exempted - until it is concluded. On Tuesday, Republican senators hinted that it would continue well past the Iowa caucuses on February 3.

Senate Majority Leader Mitch McConnell, who controls the Senate calendar, could conceivably allow the process to drag out even further just to make life difficult for the Democratic opposition.

Asked about whether the diversion would be a problem for her in Iowa at the seventh Democratic debate in Des Moines Tuesday night, Warren repeated what she has said before - that some things are more important than politics, and the impeachment of a sitting president is one of those things.

"I took an oath to uphold the Constitution of the United States of America," she said. "It says that no one is above the law [and] that includes the president of the United States. If we have an impeachment trial, I will be there because it is my responsibility."

Will the impeachment trial help Biden?

More conspiracy-minded politicians in Washington, DC suggested that Pelosi's decision to withhold the articles of impeachment was deliberate.

"If you look at the true political nature of why" Pelosi withheld the articles, House Minority Leader Kevin McCarthy said at a news conference on Tuesday, it was "to harm one campaign and harm another."

McCarthy said Biden should suspend his campaign during the trial out of fairness to the other candidates, a suggestion Democrats laughed off.

"We will not take campaign advice from Kevin McCarthy," House Democratic Caucus Chairman Hakeem Jeffries snapped in response.

Another Republican, Senator Marsha Blackburn, has gone as far as to suggest that the four Democratic senators should recuse themselves from the trial altogether.

Democratic 2020 United States presidential candidate Senator Elizabeth Warren speaks as former Vice President Joe Biden and Senator Bernie Sanders listen during the seventh Democratic 2020 presidential debate at Drake University in Des Moines, Iowa [Shannon Stapleton/Reuters]

While Biden may benefit from the extra face time in Iowa and New Hampshire, it remains to be seen whether the impeachment effort overall will be a plus or minus for Biden, who himself is partially at the centre of the impeachment itself.

His name, along with that of his son, Hunter Biden, will be at the top of Republican talking points during the case. Some have suggested the former vice president stands to gain from being portrayed as enough of a political threat to Trump to justify strong-arming an ally. The entire impetus for the impeachment revolves around Trump's attempt to get Ukrainian officials to investigate his potential challenger in the 2020 race.

But constant Republican harping about Biden and corruption and Ukraine during the hearings could also backfire. Some Republicans have gone so far as to suggest that Biden and his son should testify in the inquiry, a prospect Biden has dismissed.

"The reason I wouldn't [testify] is because it is all designed to deal with Trump doing what he's done his whole life," Biden told the Des Moines Register in a December interview. "Trying to take the focus off him. Not a single person, not one single person, not even that thug [Rudy] Giuliani and his crony compatriots have said I did anything other than my job."

About the only bright spot for Biden's challengers in the Senate is that they will be front and centre for a story that is sure to command the attention of the nation. Whether the TV face time is enough to offset the lost campaign time will only become apparent after the results are in from Iowa and New Hampshire.