Five days after finishing a disappointing third in Iowa, the former first lady trails Obama by double digits in several state polls but promised she was staying in the race until the February 5 “Super Tuesday” round of nominating contests.

For Obama, an Illinois Senator bidding to be the first black president, a win in New Hampshire would solidify his hold on the top spot in the Democratic nominating campaign and deal a second consecutive humiliating loss to Clinton, the former front-runner.

In a hard-fought Republican battle, Senator John McCain of Arizona holds a narrow lead over Mitt Romney, the former governor of neighboring Massachusetts who poured tens of millions of his personal wealth into the race.

State election officials have predicted a record turnout in New Hampshire during the most wide open US presidential race in more than 50 years, with no sitting president or vice-president seeking the nominations.

Polls close at 8 pm EST (0100 GMT Wednesday) on Tuesday, with results expected to begin rolling in quickly.

Candidates made a last bid for support on Tuesday, visiting voting stations and holding morning rallies to urge their backers to get to the polls.

“The American people have decided for the first time in a very long time it is time for change in America,” Obama told a rally at Dartmouth College.

Clinton, a New York senator, and Romney are both under heavy pressure to revive their campaigns after disappointing showings in Iowa. Until recently, Romney led polls in Iowa and New Hampshire, but he finished second in Iowa to former Arkansas Governor Mike Huckabee.

New Hampshire’s primary is the next battleground in the state-by-state process of choosing Republican and Democratic candidates for November’s election to succeed President George W Bush.

A new Reuters/C-SPAN/Zogby poll showed Obama expanding his lead over Clinton to 13 points, 42 per cent to 29 per cent, with former North Carolina Senator John Edwards in third at 17 per cent.

McCain led Romney 36 per cent to 27 per cent in the Republican race, with Huckabee at 10 per cent.

Clinton greeted a handful of supporters at a Manchester polling location before dawn on Tuesday and made similar visits in Nashua, Derry and Concord through the morning.

Asked by a reporter if she needed to win New Hampshire, she made reference to the February 5 “Super Tuesday” round of 22 nominating contests.

“I think the nominating process ends at midnight on February 5,” she said. “I look forward to campaigning across the country.”

Romney predicted the New Hampshire vote would be close and looked ahead. “Republicans are going to get behind me and independents are going to get behind me and we’re going to end up winning this thing,” he said.

The tiny mountain hamlet of Dixville Notch opened the state’s voting shortly after midnight, as it has for every election since 1960, in a balloting display more media circus than civic event.

With a room full of cameras recording their every move, a few of the town’s registered voters cast ballots. Obama won seven of the 10 Democratic votes, with former North Carolina Senator John Edwards getting two and New Mexico Governor Bill Richardson getting one.

McCain won four of the seven Republican votes, with Romney winning two and former New York Mayor Rudy Giuliani getting one.