Hillary Clinton has given her biggest hint yet that she is preparing to end her presidential bid, saying for the first time that she is open to becoming Barack Obama's vice-Presidential candidate.

The former First Lady's comments, made in New York today, come as results are tonight due to be announced from the two final primary contests for the Democratic party nomination.

Although the former First Lady is not expected to formally concede defeat to Mr Obama tonight, two senior officials were quoted as telling the Associated Press news agency that she would acknowledge that her rival has the support of sufficient delegates to clinch the nomination.

In her New York city speech scheduled for later, AP says she will pledge to continue to speak out on issues such as health care reform, which has been a key plank of her campaign. But to all intents and purposes, she will hint that her battle to become the country’s first female president is over as most campaign staff will be let go, and will be paid until June 15.

The advisers said the former First Lady has taken the strategic decision not to formally end her campaign, giving her leverage to negotiate with Mr Obama on various matters including joining him on the Democratic ticket.

That process of negotiation appeared to have started when she told congressional colleagues she would consider becoming his vice-Presidential nominee it if it would help Democrats win the White House.

In a conference call with other New York senators, she was asked by Democratic representative Nydia Velazquez whether the best way for Mr Obama to win over key voting blocs, including Hispanics, would be for him to choose Mrs Clinton as his running mate. She replied: "I am open to it," if it would help the party’s prospects in November's presidential elections against John McCain.

A statement by the campaign later clarified that Mrs Clinton was not preparing to make a formal concession of defeat tonight.

Now just 42 delegates short of the 2,118 he needs to clinch the nomination, Mr Obama was today looking to the voters of Montana and South Dakota to back him in his lengthy struggle with Mrs Clinton. While a total of only 31 delegates are on offer, the anticipated declaration of some 200 uncommitted superdelegates in the coming days - perhaps as soon as tonight - is almost certain to put him over the top. That would leave Mr Obama poised to become the first black presidential candidate for any major US political party.

While Terry McAuliffe, the Clinton campaign chairman, denied that Mrs Clinton was preparing to concede defeat, he also told NBC that “if Senator Obama gets the number, I think Hillary Clinton will congratulate him, call him the nominee."

While polling has been sparse in the two Western states, the few available surveys suggest that the young Illinois senator is on course for a healthy victory in Montana. In South Dakota, however, Mrs Clinton may snatch a surprise win. Though a mid-April poll by Dakota Wesleyan University gave Mr Obama a 12-point lead over his rival, the most recent figures suggest the former first lady has made up that ground.

In mathematical terms, even stunning victories in both states – which offer a total of 31 delegates - would fail to revive the fortunes of Mrs Clinton, who is trailing Mr Obama by some 150 delegates. However the Obama camp knows a strong performance in the two states – both predominantly white – would allay concerns about his demographic difficulties and help convince the remaining uncommitted superdelegates to rally behind him in the coming days.

Mr Obama’s advisors are predicting that enough superdelegates – senior Democrats and elected officials who are granted a free vote on the nominee – will flood to his side following the last two primary contests to allow him to declare victory later this week. Harry Reid, the Senate Majority Leader, and Nancy Pelosi, have been urging superdelegates to make their choice known this week in order to forestall a damaging fight at the Democratic convention in August and unite the party behind the nominee.

The 46-year-old first-term senator, who picked up 4.5 superdelegate votes yesterday to Mrs Clinton’s two, said he believed “that between Tuesday and Wednesday, that we’ve got a good chance of getting the number that we need to win the nomination."

There is a possibility that Mrs Clinton might still try to sway superdelegates, who can switch allegiances at any point up to the convention. However senior Democrats will no doubt work to dissuade her from pursuing that path, regarded as a nuclear option that would divide the party even further and potentially wreck its November prospects.

Bill Clinton appeared to foreshadow his departure from the campaign trail as he adopted a valedictory tone during a stop in South Dakota.

“I want to say also that this may be the last day I’m ever involved in a campaign of this kind,” he said.

“I thought I was out of politics, till Hillary decided to run. But it has been one of the greatest honours of my life to be able to go around and campaign for her for as president,” he added.

The former President issued one last parting shot however, as he attacked Vanity Fair for what he called a “dishonest” and “sleazy” story on his behaviour on the campaign trail.

The piece was part of the media’s attempt to “nail Hillary for Obama”, he said, claiming that his wife’s campaign had been subject to “the most biased press coverage in history.”

Observers were focusing on how the pair, whose fight has been a tough and frequently vicious one, might now collaborate on the Democratic bid. There has been speculation that Mr

Obama might offer Mrs Clinton a cabinet post, most probably the healthcare portfolio, in return for her support.

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