Reid Cherlin believes it's still young people:
Peter Levine, an expert in this stuff who runs CIRCLE, a youth engagement research program at Tufts University, figured out that if in 2008 the voting age had been 30—if you'd cut out 18-29-year-olds entirely—Obama would have lost North Carolina, Virginia, and Indiana altogether. "I was running the numbers," a strategist affiliated with the campaign told me, "and it was like, holy shit. If these guys don't turn out, that could be the ballgame."
Prospects don't look good, according to Zoë Carpenter. She found "the percentage of young Americans who are registered this year is the lowest of the past five presidential elections":
The race will hinge on turnout, and millennials—some 64 million eligible voters—have emerged as the main question mark among the constituencies the candidates have been courting. Romney won’t win the youth vote: The fall survey from the Harvard Institute of Politics, which polls young Americans twice yearly, shows Obama leading Romney by a 19 point margin among millennials who say they are likely to vote and by 22 points among young adults in general. For most millennials the choice is between voting for Obama, and not voting at all.
But Cherlin argues the ground game has shifted online:
Log into Twitter or Facebook or Pinterest and you can watch the gears of the reelection effort turning.
That's what you're seeing when your 19-year-old cousin shares yet another schmaltzy Obama paean, the tearjerker video of the 106-year-old lady pledging to vote for the president because he's honest, or the one of the 10-year-old whose dad came home from Iraq. It's what's happening when a friend likes an Instagram photo of rows of glistening buttons saying "Ohio Votes Early," all matching in that perfect Gotham typeface. There's the "For All" campaign, specifically targeted at the 18-29 set, asking every supporter to write on her hand "what progress means to you," put her hand over her heart, and upload the shot. Or the Lena Dunham video where she compares voting for the first time to losing your virginity; it's funny, semi-dirty, hated by conservatives, and though it may not have the power to physically transport a 20-year-old to his local polling station, it keeps him talking about Obama for another couple of dining hall meals.