WASHINGTON — Uncertainty over when the Senate will begin President Donald Trump’s impeachment trial is complicating crunch-time campaigning for the five Democratic senators seeking the White House.

Sens. Elizabeth Warren of Massachusetts, Bernie Sanders of Vermont, Amy Klobuchar of Minnesota, Cory Booker of New Jersey and Michael Bennet of Colorado are facing the prospect of having to return to Washington this week to sit as jurors if proceedings begin. In the absence of a trial, they’re filling time with hastily scheduled campaign events far from Capitol Hill.

The dynamic becomes even murkier next week, when a presidential debate in Iowa is set for Jan. 14. While that could be rescheduled if a trial prevents senators from participating, there’s limited flexibility because the leadoff Iowa caucuses are quickly approaching on Feb. 3. New Hampshire is set to hold its primary eight days later.

The confusion comes at a critical point in the Democratic contest. Several candidates are bunched at the top of many polls in early voting states, making it even more important for them to use the final stretch before voting to lock down support and try to stage a breakout victory. Instead, some campaigns are trying to hammer out schedules without knowing where candidates will be.

Warren announced a trip this weekend to New Hampshire that will feature town halls in two cities but included the disclaimer: “This schedule is subject to change depending on the schedule for impeachment in the U.S. Senate.”

“We’ll have to be flexible based on what happens,” said Bennet spokeswoman Shannon Beckham, whose candidate will be in Washington this week after wrapping up a seven-day New Hampshire swing. He has campaign events set for this weekend, at least for now.

Impeachment isn’t the only potential political firestorm muddying senators’ scheduling plans, meanwhile. Booker, a member of the Senate Foreign Relations Committee, announced he was canceling campaign stops in Iowa on Tuesday and Wednesday to return to Washington for a congressional briefing on Iran amid escalating tensions after a U.S. airstrike killed Iranian Gen. Qassem Soleimani.

Although the House voted to impeach Trump last month, Democratic Speaker Nancy Pelosi has yet to turn the articles over to the Republican-controlled Senate, where members will act as jurors in the trial and vote on the president’s guilt. Pelosi has said she wants to ensure the Senate proceeds in a fair manner — but it means that when the trial will begin, not to mention a timeline for how long it will last, is in limbo.

The timing of the trial could benefit candidates who aren’t members of the Senate. Former Vice President Joe Biden and Pete Buttigieg, the former mayor of South Bend, Indiana, have spent months crowded near the top of the Democratic field with Warren and Sanders. A trial could leave them in the early voting states without their top rivals.

Similarly, the trial could give lesser-known candidates, such as former Massachusetts Gov. Deval Patrick, a chance to make a last-minute impression with voters.

“Deval will be in the early states more than any other candidate during this critical phase of the election, sharing his record and vision face-to-face when it matters most,” said Patrick spokeswoman Aleigha Cavalier.

Warren spokeswoman Kristen Orthman said the campaign may rely on high-profile supporters to rally crowds if her candidate can’t be there. That could include former Obama administration housing chief Julián Castro, who abandoned his presidential bid last week, endorsed Warren on Monday and knows Iowa and other early voting states well, having spent months campaigning there.

In perhaps a sign of things to come, Castro is planning to attend a Warren rally Tuesday night in New York City. The senator’s husband, Bruce Mann, has also traveled with her while she campaigns in places like Iowa and has made public appearances on her behalf.

“We will be prepared for any scenario, and we are planning for any scenario including increased surrogate and spouse travel,” Orthman said.

Sanders, who has called on the Senate’s Republican leadership to move to an impeachment trial as quickly as possible, also has top supporters who have campaigned for him in the past and could do so again, including New York Rep. Alexandria Ocasio-Cortez.

Warren said she’s prepared to spend days in the Senate, then fly back to Iowa for evening campaigning “if that’s what it takes.” But that’s a tall logistical order given the limited commercial flight options between Washington and Iowa. Taking private flights could make things easier but also might undermine the working-class credibility of some Democratic White House hopefuls.

Buttigieg was criticized this fall for using private jets to travel but shrugged those off, saying, “This is a very big country, and I’m running to be president of the whole country.”