Barack Obama and Hillary Clinton continued trading blows in the final hours before tonight's Democratic presidential primaries, which polls suggest are likely to result in a split verdict.

Turnout in Indiana and North Carolina is said to be high, a reflection of how voters have been galvanised by the two candidates' epic struggle which both sides believe will now probably extend through to the beginning of June or even beyond.

Mr Obama began the day in Greenwood, Indiana, where he forecast the result of the primary is "going to be close". Although his campaign had previously identified the state as one it expected to win, he has been hit hard in recent weeks by charges of elitism, as well as the controversy over his pastor, the Reverend Jeremiah Wright.

But in recent days he has been hammering Mrs Clinton over her proposal for a "gas tax holiday", citing economists who say it is nothing more than a "gimmick" and typical of "what Washington does".

Mrs Clinton has shrugged off such charges, saying her rival is "always going on TV and he's always saying, 'oh you know it's just like $20'. Well, for a lot of people $20 is something, right?"

She added: "I believe we should start standing up for the majority of Americans who are paying the outrageous gas prices. I'm ready to take on the oil companies."

Speaking in Merillville, Indiana, she even threatened to break up the Organisation of Petroleum Exporting Countries. "We're going to go right at OPEC," she said. "They can no longer be a cartel, a monopoly that get together once every couple of months in some conference room in some plush place in the world, they decide how much oil they're going to produce and what price they're going to put it at."

Mrs Clinton has re-cast her image as an aggressively populist candidate, regularly attacking "Wall St money brokers" and claiming Mr Obama is out of touch with ordinary voters.

Polls point to a narrow win for Mrs Clinton in Indiana, where she is staging her election night rally this evening. Mr Obama will be appearing in Raleigh, North Carolina, a state where he has maintained a poll lead - albeit much reduced - over his rival.

If Mr Obama wins both contests tonight, it would probably doom Mrs Clinton's hopes, as it would send a surge of super-delegates to line up behind Mr Obama and even some of her aides to pressure her to quit.

If he loses Indiana by single digits, and wins North Carolina, it will do little to change the dynamic of the race, or the minds of most superdelegates, with Mr Obama moving steadily on toward the nomination because of his insurmountable lead among elected delegates.

If Mrs Clinton wins both, however, it will be the "game changer" she has been hoping for, dramatically increasing doubts about Mr Obama's electability and giving her a powerful case to make to super-delegates that she is the more viable nominee.

Mrs Clinton's campaign chairman, Terry McAuliffe, today predicted that she would win Indiana and declared: "She's on a roll!" But he declined to make any forecasts for North Carolina, where the large black population is expected to provide solid support for her rival.

Mr Obama has spent much of the past fortnight seeking to reduce the impact of his remarks in San Francisco last month when he suggested small-town Americans were clinging to guns and religion because they were "bitter". He has put less emphasis on the glitzy big rallies which have been a trademark of his campaign this year and spent more time at smaller events appearing to listen to the concerns of the white working class.

At a car components factory in Durham, North Carolina, he adopted an almost pleading tone with workers, saying: "I want your vote. I want it badly." Later, standing at the plant's gates, he shook hands with one man and said: "If I got your vote, it would mean a lot."

At stake tonight are a total of 187 pledged delegates - 115 in North Carolina and 72 in Indiana. The results cannot put either candidate over the 2,025 delegate total needed to win the nomination. According to Associated Press estimates, Mr Obama had 1,745.5 to Mrs Clinton's 1,608 this morning.

After today, there will be more undeclared super-delegates - around 250 - for the campaigns to fight over than the 217 remaining delegates to be earned from the remaining six primaries.

The key to the nomination is held by the super-delegates, who have continued to trickle towards Mr Obama despite his most uncomfortable month since the race began. Mrs Clinton hopes to reverse a decision to ban Florida and Michigan's 366 delegates - the bulk of which she won - because the two states broke party rules by holding primaries in January.

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