It's expected to be a race for second place. Polls showed Mitt Romney way out in front.
A spirited Republican race and good weather is helping with the turnout. ABC News' political unit reported heavy turnout among independent voters.
All the candidates will leave the state with a different appreciation for the role of demonstrators in the 2012 presidential campaign.
On Monday night, candidate Rick Santorum and wife Karen were surrounded by a pushing, taunting, sometimes screaming crowd of demonstrators as they tried to leave a campaign event in Manchester.
"Protest, say what you want. Scream and holler at me, but physical violence, there's no place for that here in this country," said Santorum.
"I was physically assaulted by one of his staffers," said fringe presidential candidate Vermin Supreme.
Supreme was in the crowd surrounding the Santorums. He blamed the candidate's security team.
"I won't call them goons, but we'll say staffers. One of his staffers gave me a shove," said Supreme.
Elsewhere, hecklers have invaded New Hampshire's famously retail campaign events. Front-runner Mitt Romney took the bait Monday.
"Hey, how about you talk, instead of shouting? Why don't you say what's your view, Madam? What do you think?" said Romney.
In Manchester, voter Karrie Stone has seen the heckling and other incidents on news reports.
"I can understand why tensions are high, I suppose. Again, we're a nation in flux," said Stone.
Various demonstrators from around the region have used the Occupy New Hampshire encampment in downtown Manchester as a base of operations. Occupier Wendy Rogers said she could not say whether candidates should expect more or less protest activity in other states.
"I think they will hear more voices. We are a very loosely knit organization. I have no idea what other states will do. This was the right thing for New Hampshire," she said.
Park Ridge man travels to NH to support Ron Paul
ABC7's political reporter Charles Thomas spoke with a Chicagoan who traveled all the way to New Hampshire from Illinois to support his favorite candidate.
Northern Illinois student Jim Blalock went by car from DeKalb to "sign bomb," as he calls it, outside a Manchester polling place this primary election day. The Park Ridge native is a Ron Paul supporter through and through.
"He moves me because he has the spirit of 1776," said Blalock as he laughed. "And when you do that, I think it brings everybody together and it excites a lot of people...We're just trying to get the word out...just trying to engage, talking to people."
The secretary of state predicts a record 250,000 voters. Sunshine and temperatures in the 40s -- balmy for New Hampshire in January -- won't hurt the turnout.
"The weather I love it. They can keep the snow," said Gene Mitchell.
"Lot of people who would normally stay in, I think, will come out and vote today," said Rebecca McElroy.
The candidates -- none of whom can vote in New Hampshire -- led their chasing media throngs to various polling places throughout day.
Former Utah Gov. Jon Huntsman took another shot at front-runner Mitt Romney's statement Monday about firing people who don't give good service.
"If you're going to make statements like that, you become pretty much unelectable, because if it isn't a Republican, it's going to be the Chicago campaign machine with a billion dollars at their sails that's going to take after comments like that," said Huntsman.
"I was talking about insurance companies. We'd all like to get rid of our insurance companies. We don't want Obama to tell us we can't," said Romney.
The question is, will the comment hurt Romney, who according to pre-election polls for several weeks now has held a commanding lead over the other candidates. Francine Grenier told ABC7 she decided on a candidate long before the firing comment controversy. When asked when she had made up her mind, Grenier said, "four years ago...Mitt Romney."
Most polls in New Hampshire close at 7:00 p.m. EST, 6 p.m. CST.