Democrat Barack Obama is on the historic threshold of becoming America's first mainstream black presidential nominee, but Hillary Clinton's camp denied reports she was about to fold her campaign.

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Voters in the last two states, Montana and South Dakota, closed out a rancorous coast-to-coast nomination duel, as the Democratic Party coalesced around its all but certain 2008 general election champion.

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With Obama on the verge of final victory, reports surfaced that Clinton was ready to admit defeat and acknowledge the Illinois senator had the nomination secured, at a “celebration” rally on Tuesday night.

But her campaign issued a statement denying the reports, and several close aides vehemently pushed back against the idea that she would quit.

“Senator Clinton will not concede the nomination this evening,” a statement from the Clinton campaign read, saying reports that she would do were wrong.

“Not true,” one aide said, on condition of anonymity, when asked whether she would throw in the towel.

Superdelegate votes

Another senior Clinton official said: “there are still people voting, and she is committed to letting everyone vote, we don't know yet how delegates will be awarded from Montana and South Dakota”.

Obama's team meanwhile nudged superdelegates – top party insiders – to jump on his bandwagon, so he would reach the magic number of 2,118 delegates and set off a raucous victory party in Minnesota later on Tuesday.

Even Clinton's most upbeat cheerleader, campaign chairman Terry McAuliffe admitted that if the mathematics favoured Obama, there was little she could do.

“If Senator Obama gets the number, I think Hillary Clinton will congratulate him, call him the nominee,” he told NBC.

“She has given her heart and soul to this campaign. We'll go through today. Let's see where the numbers are,” he said.

While an expected flood of superdelegates towards Obama had yet to materialise, there were whispers in Congress that legislators may move once polls close in South Dakota and Montana.

'Celebration' planned

James Clyburn, an influential African-American leader and a giant in Congress, endorsed Obama early today and urged other unpledged superdelegates to make their choices known.

“Today, the process ends. And I hope that enough of us will announce our intentions today so that this evening our candidate, our nominee, presumptive nominee, will be able to credit the voters of this country with having achieved a threshold of 2,118,” the South Carolina representative told NBC television.

By late early afternoon, Obama was 31.5 delegates short of the nomination, according to his campaign, with more superdelegates expected to come out later in the day.

Thirty-one elected delegates were up for grabs in South Dakota and Montana, and he was expected to win just over half of them.

Regardless, Illinois Senator Obama, 46, planned to train his full fire on potential general election rival John McCain, with a daring foray into the same Minnesota sports arena where Republicans will crown their nominee in September.

Clinton was to appear in her home state of New York, prompting speculation she might step aside at a “celebration” event in Manhattan on Tuesday night.

Morale boost

Obama, the former community organiser who has scaled the heights of US politics after just three years in the Senate, already had an eye on healing his party after a nominating duel which cleaved it down the middle.

He divulged some of the content of his congratulatory telephone call to the former first lady after her thumping win in the Puerto Rico primary on Sunday, which boosted her morale but could not dent his control of the race.

“I emphasised to her what an extraordinary race that she's run,” Obama told reporters in Michigan Monday.

At a rally in South Dakota, the former first lady said she would focus on winning over superdelegates, top party officials who can vote how they like at the party's August convention in Denver.

“Tomorrow is the last day of the primaries and the beginning of a new phase in the campaign,” Clinton said, according to the Washington Post.

Voting began at 7am in parts of South Dakota (2200 AEST) and an hour later in the rest of that state and Montana.

Polls were to close at 7pm in South Dakota (1000 AEST and 1100 AEST Wednesday), and at 8pm in Montana (1200 AEST).

Barack Obama on brink of history