COLUMBUS — The Value City Arena on the campus of Ohio State University didn’t quite fill up for President Obama Saturday morning — and that wasn’t the only sign that 2012 will be different from the heady days of 2008.

People watch a video of President Barack Obama during a campaign rally at The Ohio State University, Saturday, May 5, 2012 in Columbus, Ohio . (Haraz N. Ghanbari/AP)

Some of Obama’s supporters are nervous — really nervous, even physically anxious — at the prospect of him losing after a single term.

“I’m kind of freaking out,” said Grace Jones, 35, a self-described “chronic student” who was thrilled to be in the same building as the president but also jittery about the difficult political climate.

“I don’t want to even say it,” Jones said, “but a few really good dirty tricks by the other side, and he could lose.”

Jones wasn’t sure how much she would volunteer for Obama next year. She’s afraid to invest too much emotion in a year when his re-election seems so much more uncertain than four years ago.

Still, those sorts of jitters may serve the campaign well at a time when the president is trying to impart a sense of urgency about about how close the election could be. In addition to drawing his most stark contrast to date with his opponent Mitt Romney, Obama and first lady Michelle Obama made a point of urging the crowds in both Columbus and Richmond Saturday to get involved and give as much time to the campaign as possible.

“Think about it,” Michelle Obama said. “It could all come down to those last few thousand folks who get out to the polls on November the 6th. And when you average that out over this entire state, it might mean registering just one more person in your town. It might mean helping just one more person in your community get out and vote on Election Day. So know this: With every door you knock on, with every call you make, with every conversation you have, I want you to remember that this could be the one that makes the difference. This could be the one.”

The crowd that filed into the stands in Columbus in anticipation of Obama’s general-election kickoff rally was mixed. Supporters included parents and their small children, the middle-aged and the elderly, and a healthy representation of minorities. Despite the campaign’s strategy of staging the high-tech rally on a college campus, with lots of electronic outreach on social networks, the arena was not teeming with an overrepresentation of college students.

That fact underscored a central challenge Obama faces this year: to excite the younger generation of voters that helped push him to victory four years ago..

Some of those younger voters expressed a slightly gloomy view of the election, too.

“I’m excited because I get to vote for the first time,” said Emily McDaniel, 17, of Westerville, Ohio, who will turn 18 before the election. ”But I’m scared of the slight chance he isn’t re-elected.”