Bernie Sanders arrives at the Capital Hilton to meet with Hillary Clinton in D.C.

June 14, 2016 Bernie Sanders arrives at the Capital Hilton to meet with Hillary Clinton in D.C. Matt McClain/The Washington Post

The senator from Vermont is Hillary Clinton’s rival in the contest for the Democratic presidential nomination.

The senator from Vermont has become Hillary Clinton’s chief rival in the contest for the Democratic nomination.

The senator from Vermont has become Hillary Clinton’s chief rival in the contest for the Democratic nomination.

— In coming days, students who surf the Web here at Iowa State University are likely to encounter a digital ad running along the side of their screens. “Hey Cyclones!” it says. “Head Home For Bernie.”

It may be a tall order.

The invitation by the Democratic presidential campaign of Bernie Sanders is meant to maximize the impact of one of his strongest constituencies in his battle against Hillary Clinton: young voters. But four days ahead of the Iowa caucuses, it also exposes one of his central vulnerabilities — the difficulty of drawing those young voters to the polls.

Adding to the challenge are the complicated rules of Monday’s caucuses, where all votes do not have equal impact. Much of Sanders’s support in Iowa is concentrated in the state’s college towns. There is strategic advantage in persuading college students to vote in the communities where they grew up rather than on campus, to spread out his support across the state and win more delegates.

But can these voters be bothered?

The Fix's Chris Cillizza previews the Iowa caucuses, looking at what the outcome could mean for both Democrats and Republicans. (Peter Stevenson/The Washington Post)

“I think college kids want things to be as easy as possible,” said Ryan Thornton, a junior majoring in mechanical engineering at Iowa State who counts himself as a Sanders supporter and plans to caucus on campus.

Like other students interviewed for this story, Thornton, whose home is about 90 miles north of school, was skeptical that his peers would take the Sanders campaign up on its offer.

“On a Monday night, for me personally, that’s a 2 1/ 2 -hour drive I’d have to make,” said Alex Doser, a senior majoring in mathematics, a Sanders supporter and president of the Iowa State University College Democrats. “I think that’s going to be a hard thing to get students to do for the most part.”

The effort is one of numerous ways the candidates are trying to game the system. In this case, it underscores how crucial the turnout of millennials is to the fate of Sanders’s bid. The unreliable nature of their vote is one of the reasons the outcome of Monday’s key contest remains so much in doubt.

[Sanders: Clinton is running a ‘desperate’ campaign’]

Some of Sanders’s biggest and most boisterous crowds on the campaign trail have been at rallies in college towns, and his campaign says it now has operations set up on 38 campuses across Iowa to boost turnout.

A Quinnipiac Poll this week was among the latest to show the race between Clinton and Sanders to be a statistical dead heat in Iowa. But among likely caucus-goers under age 45, Sanders held a commanding lead, 78 percent to 21 percent.

Democratic presidential candidate Bernie Sanders speaks to students at Roosevelt High School during a campaign event Jan. 28. (Joshua Lott /For The Washington Post)

Sanders’s aides freely acknowledge that they’ll need a big showing from those voters — as well as some other demographics that don’t typically show up in large numbers — to topple Clinton.

“The polling shows, and the evidence on the ground bears out, that both campaigns have enough people to win,” said Jeff Weaver, Sanders’s campaign manager. “The challenge is to get your people out.”

To that end, the campaign is also making a big push among high school seniors. Under the caucus rules, 17-year-olds are allowed to participate if they will turn 18 by the November election.

On Thursday, Sanders held his latest in a series of forums at a high school, appearing in a packed auditorium at Theodore Roosevelt High School in Des Moines. He offered a ready-made response for anyone who was mocked by their peers for caring about politics.

“Tell them, ‘I am fighting for the future of my country, and if you’re not concerned about that, you’re the dumb guy, not me,’” Sanders said.

The Clinton campaign is also trying to make sure its younger supporters turn out but is pursuing a different approach. Michelle Kleppe, the campaign’s organizing director, said the Clinton operation is relying on people with deep roots in the community to mobilize supporters of all ages.

[In Iowa’s final stretch, Clinton pivots away from attacking rival Sanders]

The Sanders campaign’s gamesmanship regarding college students underscores one of the quirky aspects of the Iowa caucus system. The goal of the night is to win delegates, which are awarded based on the results in each of the state’s 1,681 precincts.

Each precinct has a set number of delegates available, and in some cases, a campaign will get the same number of delegates whether they win by 10 votes or many times that. Given how strong Sanders is in college towns, there’s an incentive to send some of those supporters to precincts where he’s more in need of the help.

A Des Moines Register/Bloomberg Politics poll this month showed that Sanders’s supporters are disproportionately concentrated in several counties with large college campuses.

Pete D’Alesandro, Sanders’s campaign coordinator in Iowa, said the campaign is not leaning on college students to head home — but is making it an option.

“It’s not a matter of enticing a kid,” said D’Alesandro. “We’re encouraging them to caucus where they’re most comfortable. The important thing is that folks don’t wind up falling through the cracks and just sitting in their dorm room.”

The digital ads — which are tailored to individual campuses — invite students to click on a “Learn More” button. At that point, students are given two options to select: “I will caucus for Bernie in my home town” or “I will caucus for Bernie at my school.”

Under both scenarios, students are asked for contact information and whether they’ll need a ride on caucus night.

The ads also make an appeal based on the reputation for apathy among students: “They say you don’t care. They say you won’t caucus. They say Bernie can’t win. Prove them wrong.”

The campaign has even set up a separate website — provethemwrongandcaucus.com — playing on that sentiment.

Dennis Goldford, a political science professor at Drake University, said the Sanders “go home” strategy is largely untested but makes sense to try given the intensity of his college support.

“Is this a Hail Mary? No,” he said. “I think it’s a reasonably grounded turnout strategy.”

In 2008, then-Sen. Barack Obama (D-Ill.) benefited from an unusually strong turnout among young voters, a dynamic Sanders is trying to replicate. But the calendar was different that cycle, Goldford said. The caucuses were held nearly a month earlier, on Jan. 3, a time when most students were still home on winter break.

As a college professor himself, Goldford said he will cut students some slack on Monday if they miss class. Such reassurances might not be enough for some Sanders supporters, including John Fisher, a senior at the University of Northern Iowa in Cedar Falls, who is also a student teacher.

“I have a lot of stuff to do at night,” said Fisher, who’s majoring in math education. “If I spend three hours driving, I might not get it all done.”