The Iowa caucuses are happening in a little less than two weeks, and they’re on a collision course with a certain event that’s keeping several top candidates stuck in Washington: President Donald Trump’s impeachment trial.

Three of the candidates polling in the top five in Iowa are sitting US senators — Bernie Sanders (I-VT), Elizabeth Warren (D-MA), and Amy Klobuchar (D-MN). And all three plus the lower-polling Sen. Michael Bennet (D-CO) will have to substantially curb their time on the campaign trail starting Tuesday.

They’ll be stuck in the US Senate chamber for six days a week, and as Senate jurors, they won’t be able to talk during the trial; the best they can do is ask written questions and tell reporters how they think it’s progressing.

Campaign spokespeople for three out of the four senators told Vox they’re planning to fill in with a range of tactics such as having high-profile surrogates and candidates’ families hit the trail in the early states this week. And with social media, senators can hold events remotely or send messages out to their supporters.

Still, all of these are poor substitutes for consistent, on-the-ground campaigning. This critical last stretch before Iowa hinges on convincing the nearly two-thirds of voters who are undecided to support you; no matter what, losing out on critical face time with these people is a blow.

“Of course it matters,” Warren told Politico’s Burgess Everett recently. “We just did a 3½-hour selfie line. Don’t tell me it doesn’t matter to do face-to-face.” Sanders, too, expressed his concerns. “I would rather be in Iowa today. ... I’d rather be in New Hampshire and Nevada and so forth,” he said last week.

Both candidates, however, have stressed that it’s their constitutional responsibility to be present as jurors for the impeachment trial. “Some things are more important than politics,” Warren said during last week’s debate.

For Warren and Sanders in particular, the ongoing trial means ceding valuable campaign time to the two top candidates who are not in the Senate and have no official duties: former Vice President Joe Biden and former South Bend, Indiana, Mayor Pete Buttigieg. And it could take the wind out of Klobuchar’s sails in particular; since she’s polling fifth and in the single digits, she can’t afford to lose much time with voters.

“You can’t do this by proxy or media,” said Patrick Murray, director of the Monmouth University Polling Institute. “It does require the candidate to be on the ground; that’s what voters expect. So will they give them a pass, understanding they need to be in Washington? It’s a big question mark.”

How the senators’ campaigns are hoping to fill in in the blanks

For about a week, voters in Iowa and New Hampshire should expect to see a lot of the candidates’ spouses and family, as well as high-profile surrogates who have endorsed them.

Warren and Sanders, in particular, have a number of celebrity endorsers who could hit the trail to draw crowds. A Warren campaign spokesperson pointed Vox to a previous statement made by communications director Kristen Orthman to the Washington Post.

“We are looking at all options,” Orthman said, floating the possibility that Warren’s husband, Bruce Mann, a Harvard Law School professor who has kept a fairly low profile so far, would get out on the stump for his spouse. Orthman added that Warren could also do remote appearances to continue to have contact with voters from Washington.

Likewise, Klobuchar said previously she would have her family and local surrogates in Iowa and New Hampshire hit the trail on her behalf. There’s also the possibility the senator will conduct tele-town halls to connect with supporters.

“My husband loves to campaign,” Klobuchar said during a recent appearance on The View. “My daughter loves to campaign, she’s pretty good at it.”

Sanders has a number of high-profile political and celebrity endorsers who have already hit the trail for him, including actors Danny DeVito, Mark Ruffalo, and Susan Sarandon. Later in the week, progressive Rep. Alexandria Ocasio-Cortez (D-NY) will stump for the senator in Iowa. Sanders had initially planned to hold a night-time rally Wednesday in Iowa, but it was cancelled due to the impeachment schedule.

Some candidates have fewer options.

Klobuchar, who cannot afford anything other than overperforming expectations in the Iowa caucuses, could experience serious setbacks from losing a week on the trail.

“Klobuchar has a decent organization, but because she’s on the outside looking in and is making small gains, [she’s] the one who could end up having her chances hurt the most,” Murray said.

Still, she benefits from having her national campaign operation in Minnesota — a little over three hours from Iowa — meaning organizers and volunteers can blanket the state while she’s in Washington, DC. Klobuchar’s local surrogates in New Hampshire are planning to do the same.

The timing also poses a major challenge for Bennet, who has struggled to gain traction.

Currently polling at less than 1 percent nationally, according to the RealClearPolitics average, he is among those who have significant ground to make up in order to have a shot at viability in Iowa and New Hampshire. Despite the conflict the trial will pose, his campaign notes that it will stay focused on New Hampshire and hold events there when the senator is not required to be in the chamber.

“He is holding 50 town halls in New Hampshire in the final weeks before the primary, answering every last question, to show why he is the best candidate to take on and beat Donald Trump,” a spokesperson told Vox.

Murray said he could see Sanders and Warren with the best chance of survival; Sanders has an extremely loyal supporter base, while Warren has a highly organized campaign operation in the early states that will still be going door-to-door even when she’s not in the area.

“Candidates that have a very good organization like Elizabeth Warren may be able to weather this without too much of a hit, but we really don’t know — the question mark is so huge,” Murray said.

Biden and Buttigieg, the other two frontrunners, are able to operate without the constraint of impeachment timing — and can instead traverse these states to talk with voters about issues like health care, climate change, and the economy. This logistical advantage has even prompted Republican Sen. John Cornyn to hypothesize that this dynamic would be a boon for Biden. “It’s pretty clear that the candidate who stands the most to gain from their absence is former Vice President Biden,” Cornyn said in a statement.

Impeachment is pretty low on the list of Iowa caucus-goers’ most pressing issues, according to the most recent Des Moines Register/CNN/Mediacom poll conducted by veteran pollster J. Ann Selzer. Selzer’s poll found impeachment coming in dead last in issues Iowa caucus-goers thought were “extremely important”; climate change and health care were tied for first.

“You’d expect the candidates who could be on the ground the whole time to build an advantage,” Murray said. Beyond the top-tier candidates, other lower-ranking candidates who have gained momentum in the early states like entrepreneur Andrew Yang and billionaire activist Tom Steyer could also capitalize on this time and use it to their advantage.

Not all voters need to see candidates up close before deciding

Iowa and New Hampshire have a heavy emphasis on retail politics, but it’s worth noting that not everyone goes to rallies or town halls before deciding whom to vote for.

“There are a large number of caucus-goers in Iowa, primary goers in New Hampshire who don’t see a candidate,” Murray told Vox. Polls he conducted on the two states this fall found a minority of voters had already been to an event. As of September, just 22 percent of likely Democratic primary voters in New Hampshire had seen a candidate in person. That number was higher in Monmouth’s November poll of Iowa, where 34 percent of likely caucus-goers had seen a candidate. Still, another 42 percent of people in Iowa told Monmouth they didn’t plan to see a candidate up close and personal.

And at least ahead of New Hampshire, candidates will have one more debate during which they can attempt to reach voters and advocate for their campaigns.

Given how fluid the race still is in both states, though, the need for some last-ditch campaigning certainly seems paramount. As of last week, roughly two-thirds of voters in Iowa and New Hampshire still hadn’t made a final decision on which candidate to support.

More time meeting with voters is just one of the ways 2020 Democrats could ultimately influence that decision. And it’s a big one.