US elections: Obama and McCain storm ahead as Hillary admits: 'I'm the underdog now'

White House hopeful Barack Obama today stormed ahead of his increasingly flagging rival Hillary Clinton as voting in the New Hampshire primary began.

The former First Lady choked back tears as she acknowledged her one-time clear lead had been wiped away and was now an underdog in the campaign to become the Democrat's candidate in November's presidential election.

In the Republican race, Senator McCain has a lead over main rival Mitt Romney.

Candidates are hoping to gain momentum before 20 states hold their elections on February 5, known as Super Tuesday.

Two tiny hamlets, Dixville Notch and Hart's Location, traditionally the first to vote in New Hampshire, opened their polling stations at midnight, giving small, early victories to Mr Obama and Mr McCain.

Most of the rest of the state began voting at 6am local time (11am).

Polls close at 8pm (1am Wednesday) and first results are expected shortly afterwards.

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New York Senator Clinton, Rudy Giuliani and Mike Huckabee were among the candidates who turned up at polling stations as they opened in a last-minute bid for votes.

After a disappointing second-place showing behind Mr Huckabee in last week's Iowa's caucuses - the opening battle of the primary season - Mr Romney needs a strong showing in New Hampshire to keep his campaign alive.

"Right now it's a neck-and-neck race," he told reporters in Stratham, New Hampshire, on Monday.

"But with the debate last night and the support I received from that debate, I anticipate winning tomorrow."

Opinion polls indicate that Mr Huckabee, a former Arkansas governor and Baptist minister, is polling third.

Both are threatened by a resurgent Mr McCain who after seeing his campaign nearly derail last summer, has concentrated much of his time and money on New Hampshire.

Other Republican candidates have been looking beyond New Hampshire primary to focus on states that go to the polls later.

Former New York mayor Mr Giuliani has been focusing on Florida's January 29 contest, hoping to gain momentum going into Super Tuesday.

Actor and former Senator Fred Thompson has been campaigning in South Carolina, where Republicans vote on January 19.

Humbled Mrs Clinton's voice trembled with emotion as she told a group of undecided New Hampshire voters in a coffee shop that politics for her was personal.

"It's not easy, and I could not do it if I just didn't passionately believe it was the right thing to do.

"I have had so many opportunities from this country, I just don't want us to fall backwards," she said.

She has seen her lead in the polls eclipsed by Mr Obama, who told cheering supporters in Claremont, New Hampshire, that "you're the wave and I'm riding it".

Analysts say New Hampshire's large bloc of independents - about 45% of registered voters - could be key to swinging the primary.

Mr Obama, in particular, showed strong appeal at Iowa's caucuses among such voters, who are registered as neither Republican nor Democratic.

Despite a USA Today/Gallup poll indicating a 13 per cent lead for Mr Obama, Mrs Clinton vowed to "keep going until the end of the process on February 5 " no matter what the result of the New Hampshire vote.

While Iowa and New Hampshire - two of the earliest contests - do not necessarily produce a winner from either campaign, they can boost contenders' chances ahead of primaries in larger states.

Bookmakers Ladbrokes have McCain as the third best bet after Obama and Clinton to be next president, giving him odds of 6/1, with Giuliani fourth at 7/1 and Huckabee at 8/1.