A sidelight to the results of the New Hampshire primary on Tuesday will be whether the much elusive youth vote, that demographic of 18-24 years olds, turns out in droves.

They did so in Iowa, and to the benefit of Barack Obama — surprising many who doubted that they would bother to show up to the caucus, based on past history.

“Now you have a confluence of candidates, organization and passion,” says Marc Morgenstern, executive director of Declare Yourself, the non profit founded by Norman Lear targeting registration among young voters.

He sees New Hampshire, with a younger population, as a testing ground to see if an even broader section of the young electorate turns out, not just those who already are politically involved.

There’s been reason for skepticism: In 2004, the Kerry campaign in particular pumped up the fact that so many young people had been registered to vote for the first time. Declare Yourself even registered 1.2 million, helping to boost participation among those 18-25 by 11% from 2000. But only 47% in that age group voted, compared to 66% of those over 25.

But Morgenstern sees encouragement from the Iowa results: Almost a quarter of the Democratic caucusgoers were under 30, compared to 17% in 2004, according to entrance polls from the firm of Edison Media Research. Obama drew 57% of voters 17 to 29, and Mike Huckabee drew 40%.

He expects to see even better results in New Hampshire, where campaign insiders tell him “they are seeing numbers than will exceed Iowa.”

Morgenstern says that younger voters are being inspired by the tightness of the race, the organizing ability of campaigns and candidates who are “igniting the passions of a lot of the young in a way that we haven’t seen since the 1970s.”

That was when the voting age was lowered from 21 to 18, but there also was a Vietnam war draft.

Declare Yourself is using celebrities like America Ferrara and Hayden Panettiere to promote registration, and so far about 250,000 have done so on their website. Their goal is 300,000 by “tsunami Tuesday” on Feb. 5, and 2 million by election day. Other viral video spots are irreverent, like one featuring Ed Helms from “The Office.” “You can’t preach to this ironic generation,” Morgenstern says. “You have to nudge them by using humor.”

Although social networking sites like Facebook and MySpace have devoted time and attention to the election, Morgenstern says that in certain cases it takes much more than social networking to get young people interested in the campaigns.

“It takes more pressure from your peers and more attention from celebrities and more passion from the candidates themselves,” he says.