Charlie Hoffman was canvassing outside a University of Wisconsin-Madison library when he heard it. The 21-year-old campus College Republicans chairman had been passing out pamphlets about the upcoming gubernatorial race -- in which incumbent Republican Gov. Scott Walker was maintaining a 1 point lead over Democrat challenger Mary Burke -- and encouraging passers-by to vote in the midterm elections. Then, a student stopped, confused. “What election?” she asked.

There's a problem with midterms and millennials. Only about a quarter of eligible millennial voters -- defined as people between the ages of 18 and 29, who grew up during the new millennium -- said they planned to vote in Tuesday's midterm elections, according to a Harvard University Institute of Politics survey released last week. The actual turnout might be even less. In 2010, 31 percent planned to vote but only 23 percent did.

These numbers don’t come as a shock. About half of all eligible young voters turned out to support Barack Obama in 2008 and 2012. But “historically, the numbers show that college-aged students don’t really turn out when it comes to midterm elections,” Hoffman said.

Midterm participation is typically low across the board, regardless of demographic. But young people are particularly disengaged. Experts have come up with lots of theories: Millennials are overlooked by campaigns, they can’t figure out how to register, they simply don’t care enough, or, as in the Wisconsin student’s case, they just don’t know the midterms are happening.

This demographic presents an opportunity for politicians -- if they can find a way to connect. Millennials are the largest and most racially diverse generation in the country’s history, making theirs the vote to get. By 2015, they will make up a third of the electorate, and in the meantime, their views are changing. No longer reliably liberal, they could swing several races.

Although about half of millennials who planned to participate said they’d vote for Democrats in the midterms, another third said they’d choose Republicans, according to Fusion’s Massive Millennial Poll. The Harvard poll found even more of a shift: Of millennials who said they would “definitely” vote, 51 percent preferred a Republican-run Congress. "Just like their elders, the younger generation are divided in many ways: by race, class, gender,” said Kay Scholzman, a political science professor at Boston College. “There’s no particular reason to expect they will all vote the same way.”

Their fickleness can undermine outreach efforts. Midterm campaigns are much smaller than presidential ones, and parties direct their resources to dependable voting blocs, said Rock the Vote President Ashley Spillane. “There’s not as much energy expended educating and appealing to young people to get involved,” she said. “It’s a disservice to the country to lose that representation.”

Another obstacle: Voting requires a predictable address, if not a permanent one. Because much of the millennial demographic is nomadic, temporarily living at college and moving around the country for jobs, they may not understand (or want to deal with) the voter registration process, said Elizabeth Matto, director of the Eagleton Institute of Politics' Youth Political Participation Program at Rutgers University. Half of the Harvard poll respondents said they had not seen any voter registration materials at work, school or in the community.

Some states have laws that actively discourage youth voters. On-campus early-voting sites are vanishing in North Carolina. In Texas, polling locations will accept concealed handgun licenses as proof of identity but not college-issued student IDs.

Education is also to blame for low turnout, Matto said. Baby boomers and Generation Xers took government classes when they were young, but now “it’s getting more and more challenging to find civics classes or any kind of education in elementary and secondary schools about the political process,” she said. Millennials show a strong sense of social responsibility. Deloitte found 43 percent of them actively volunteer, and 63 percent donate to charities. But they apparently don't see voting as an important civic duty. Eighty percent of millennials do not consider themselves politically engaged or active, according to the Harvard poll.

The solution is mobilization, said Peter Levine, director of the Center for Information and Research on Civic Learning and Engagement. “There’s a lot of young voters, and they will vote if they’re contacted and mobilized,” he said.

Doing so was a priority for Obama and the Democratic Party in 2008. Their strategy was to talk to young voters about issues in the same way they addressed older voters, said Democratic National Committee press secretary Michael Czin. Meanwhile, the GOP had virtually no presence on college campuses, but the Republicans have recognized their mistake.

“In 2008 and 2012, we saw the Democrats (especially Obama) have great success on college campuses because the Republican Party was not engaging on college campuses the way we should have been,” said Raffi Williams, deputy press secretary for the Republican National Committee. In response, the GOP created its Campus Captains program, which appoints student leaders to educate and motivate voters.

The Republicans’ youth mobilization effort improved with Mitt Romney, and Levine said he looks forward to 2016. “It’ll be an interesting new playing field since Barack Obama won’t be on the ballot, and in particular, the Republican Party will have a new cast of characters,” he said.

Both parties developed their midterm outreach strategies around meeting the voters where they are. In millennials’ case, that meant posting on social media sites such as Twitter, Facebook, YouTube, Reddit and Instagram. But even those efforts were hit-or-miss.

The GOP got smacked for an ad in the style of the TV show "Say Yes to the Dress" that equated choosing a candidate with shopping for the perfect wedding gown. In the ad, young, female voters decide to pick a Republican candidate, to the chagrin of their older mothers who like the Democratic style. Critics derided the attempt to connect with young women as sexist and condescending.

Obama wrote an essay on the website Medium, encouraging millennials to take charge of the economy, but critics argued that young people rarely visit that site. "Someone Should Tell Obama Medium Isn't Cool," wrote one observer.

Politicians from both parties make campaign stops on college campuses. Other organizations encourage voting overall: At North Carolina State University, a party bus with snacks and models will shuttle students to vote as part of a Cosmopolitan magazine prize. Overall, though, the key to getting millennials to vote will be to engage them on issues: 28 percent of respondents in the Fusion poll said the economy was the most important issue facing the U.S., while 16 percent said debt and spending were the most pressing -- about the same amount cited terrorism and national security.

But when asked if political engagement or community volunteerism was the best way to fix important issues facing the country, 42 percent of Harvard poll respondents said volunteerism. Another 37 percent said they weren’t sure. Only 18 percent said political engagement.

“Young people are far more interested in issues and solving problems than they are in partisan politics, and the entire election is set up as a partisan fight,” Spillane said. “There’s a disconnect between ‘This is an issue I care about so I must vote.’”

Rock the Vote’s Turn Out For What campaign is working to emphasize the link between voting and changing laws. The organization shot a music video with celebrities saying why they’re going to the polls next week: Lil Jon wants to support marijuana legalization; Lena Dunham is protecting reproductive rights; Darren Criss is interested in improving education.

Those issues will likely be the same ones driving discussions in 2016, said 21-year-old Hayley Young, the College Democrats chair at the University of Wisconsin-Madison. And when that rolls around, will candidates need millennials to win? Czin didn't miss a beat: “Without a doubt, yes.”