Polls favor Senator Obama in North Carolina, though the race has tightened in recent days, after he suffered through a miserable April. -- PHOTOS: AFP, REUTERS

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A heavy turnout was reported in the contest for four electoral votes up for grabs in the US territory's nominating caucuses, held more than 8,000 miles (12,900 kilometres) from Washington.

Local Democrats said some 4,000 people could take part in the Guam caucuses, up from 1,500 in 2004, highlighting the global interest in the rivalry between the pair, neither of whom made it to Guam.

They gave interviews to local radio stations as they traded new blows in the chase for voters in Indiana and North Carolina states, which hold crucial primaries on Tuesday.

Obama and Clinton concentrated their efforts on the hot-button domestic issue of soaring gasoline prices.

The former first lady promised said Indiana and North Carolina would be a defining moment in the Democratic race in which Obama has a hefty mathematical advantage.

'This primary election on Tuesday is a game-changer, this is going to make a huge difference in what happens going forward,' the New York senator declared, as she rallied supporters at a tractor showroom in North Carolina.

Polls favor Senator Obama in North Carolina, though the race has tightened in recent days, after he suffered through a miserable April.

The race was too close to call in midwestern Indiana as the rivals dueled through the final nine nominating contests of a 16-month battle for the right to take on Republican presumptive nominee John McCain in November's presidential election.

'I have no doubt that these are going to be tight races,' Obama told reporters in Indiana, bemoaning a 'rough couple of weeks' after his loss to Clinton in Pennsylvania and a new controversy over his fiery former pastor.

Clinton pummelled Obama over his opposition to a 'holiday' on gasoline taxes as US drivers are hit by high prices at the pump - in many places as high as 3.5 dollars a gallon.

Later, she used a speech at the North Carolina Democratic Party's annual Jefferson-Jackson Day Dinner in Raleigh to portray herself as a workers' champion, promising to push for jobs, health care and an end to the war in Iraq.

Obama says the gas tax moratorium would save most Americans less than 30 dollars over the summer.

His campaign, which accused her of 'pandering', debuted an political ad titled 'Pennies' decrying Clinton's plan, even as she introduced the proposal as a Senate bill.

Clinton is trying to convince 'superdelegates' - nearly 800 top party officials who hold the balance of power, that neither she nor Obama can reach the 2,025 delegate threshold to win outright, and that she is the best choice.

A tally by independent website RealClearPolitics.com had Obama up by 139 total delegates, 1,738 to 1,599, and fast closing on his rival's current 266-249 lead among superdelegates, with less than 300 superdelegates undeclared.

One undeclared superdelegate, representative Susan Davis from California, said that Clinton and Obama both had a responsibility to come together to unite the Democratic whoever emerged as the party nominee.

'I don't believe that either of the candidates who are the top nominees of the Democratic Party now should quit the race.'

'They are injecting such enthusiasm among new voters in this election.'

Polls meanwhile showed softening support for Obama.

A Howey-Gauge poll in Indiana had him barely ahead, 47 per cent to 45.

Clinton trailed by 15 points in the same poll in February.

A Rasmussen poll in North Carolina on Friday had Obama leading by nine points, but with Clinton closing, cutting the gap from 14 points earlier this week.

A Zogby tracking poll on Friday had Obama up 16 points in North Carolina, where he hopes his coalition of African-Americans, affluent white voters and students will carry him to a clear victory. -- AFP