PHILADELPHIA — President Obama’s re-election campaign tonight kicked off its first major, public event with prospective voters, going directly to the source of Obama’s most passionate support in 2008: college students.

To the thumping beat of a DJ’s tunes, more than 300 students crowded the Hall of Flags at the University of Pennsylvania for a pep rally, panel discussion and chance to see Obama campaign manager Jim Messina, who was making his first public appearance on the campaign trail.

Messina, flanked by Philadelphia Mayor Michael Nutter and Obama policy adviser Melody Barnes, laid bare what the campaign has been calculating behind the scenes – that younger voters, particularly first-timers, will be essential to Obama winning in 2012.

“What people don’t focus on is there’s 8 million voters who are 18 to 21 who weren’t old enough to vote last time and who are going to cast their first vote for Barack Obama,” Messina said, making the case for what he said would be a “historic” grassroots push for the president.

“Your older brothers and sisters started it, and you’re going to complete it,” he added to spirited applause.

The campaign billed the hour-long program as the Obama Student Summit, clearly aimed at raising the stakes for millennials and rekindling their enthusiasm ahead of an election now just 368 days away.

Barnes, who was introduced as a “Democratic strategist” since she is still officially part of the White House staff, prosecuted the case for Obama’s jobs plan, health care overhaul, and initiatives for higher education and student loans, insisting that if a Republican is elected the popular policies would all go away.

“When you leave your campuses, do you want to walk into your professional lives in a country that’s leaning forward or one that’s leaning backward?” she asked, outlining the familiar contrasts between Obama’s policies and those of Republicans.

“Those are the choices that will be made every day, and especially a year from now in 2012,” Barnes said.

There was little talk of the lagging economy and jobs crisis that has left a disproportionately high number of recent college graduates without work — and less enthusiastic about politics and Obama than four years ago, according to recent polls.

While Obama won the overwhelming majority of voters under 30 in the 2008 election, taking 66 percent to John McCain’s 32 percent, he now faces challenge of getting them to turn out again.

“Last time, there was a lot more need for change, and there has been a lot of controversy with what Obama’s been doing recently,” said Girish Balakrishnan, 23, an animation and design student at Drexel University. “So it’s iffy to me if he’s even done that great of a job, so it’s always nice to see perspective.

“I’m still on the fence. … At this point, I don’t know who to even vote for,” he said. “But there’s still a lot of time.”