Armed with “Kiss Me I’m Pro-European” stickers and bundles of leaflets, students from across the north of England gathered at a university for training on honing “elevator pitches” and deploying statistics in debates.

Their mission: to mobilize other young people—the most pro-European segment of the British population, but also the age group least likely to vote.

With just weeks to go before a June 23 referendum, Prime Minister David Cameron and others lobbying for the U.K. to remain in the European Union face a challenge: getting young people to go to the polls. With surveys showing the vote likely to be close, turnout will be critical.

The government and other pro-EU campaigners have been intensifying their focus on young voters. In a recent speech, Mr. Cameron warned that university students and those starting their careers will be hardest hit by economic shocks if Britain exits the EU.

Those pushing for a so-called Brexit are rolling out their counterstrategy. The campaign group Vote Leave said it has been making its case in about 80 university campuses through its Students for Britain wing.

Vote Leave says future generations will be better off outside the EU because Britain would redirect the money it currently contributes to the bloc and spend it on domestic priorities instead.

But herding students to the voting booth could be difficult for either side.

By June many have left university to start their summer vacations. Many have registered to vote in the cities where they study, rather than their hometowns.

There are also a host of distractions. The referendum coincides with the European soccer championships and the Glastonbury Festival, one of the U.K.’s biggest annual outdoor music events. The June 22-26 festival is expected to draw about 200,000 revelers.

Challenged by a student about the scheduling conflict, the prime minister urged those planning to attend Glastonbury to vote by mail instead. He added that he always liked to watch the summer festival on television at home “in front of a warm fire.”

Surveys suggest those under the age of 30 are overwhelmingly in favor of remaining in the EU, while most people aged 50 and over say they would like the U.K. out.

But historically, turnout has been far higher among older voters—82% for those aged 65 and over in last year’s general election, against 48% for the 18-to-24-year age group, according to the British Election Study.

The pro-EU camp plans a major, two-week registration drive in early May—just after local elections and before the summer break. Their plan includes distributing leaflets, a campaign bus touring campuses and enlisting celebrities to back the cause.

They hope for a crescendo of coordinated campus parties coinciding with the May 14 final of the Eurovision Song Contest—an annual event that has long inspired student drinking games.

In addition to encouraging them to turn out, they are also hoping young people will help convince their older relatives, hoping to replicate a pattern seen in last year’s Irish referendum on same-sex marriage.

To that end, the Britain Stronger in Europe group has launched a “Talk to Gran” campaign, which encourages younger voters to send an email card to an elder relative asking them to vote to stay in the EU. The prepared e-cards, addressed to “Dad,” “Granddad,” “Gran” and “Mum,” sign off with “I promise I’ll be back for Sunday lunch.”

During the six-hour boot camp in Manchester, student activists practiced how to brush off anti-EU debaters, cold-call voters and get articles into student newspapers. They received instruction on delivering 30-second pitches and warnings about getting sucked into arguments on the street.

Debating tips were also shared. Don’t just quote statistics from institutions like the International Monetary Fund, the students were told. “Most people, they are like, ‘Who are the IMF?’” said one adviser.

The campaigners have their work cut out for them. Outside the university bar, 20-year-old computer sciences student Abdul Abdul said he wasn’t aware of the coming referendum. “I just focus on going to university,” he said.

Hayley Shaw said she intended to vote to remain in the EU because she liked its policies on climate change but hadn’t focused on the logistics. “I keep meaning to find out when the vote is,” said the 20-year-old ecology student.