Hillary Clinton dramatically changed tactics in a bid to boost her ailing White House campaign today by launching a fierce attack on Barack Obama, accusing his campaign of "shame" for spreading information about her.

Days after appearing to strike a conciliatory note with her main rival for the Democratic candidacy when she said she was "honoured" to appear alongside him in a debate, Ms Clinton last night told a rally in Cincinnati, Ohio, that he had deliberately misrepresented her positions on healthcare and trade in mail shots to voters.

"Shame on you, Barack Obama," she said, clutching two of the mailings in her hand.

Referring to tactics used by Karl Rove, former top adviser to President Bush, she added: "Enough with the speeches and the big rallies and then using tactics that are right out of Karl Rove’s playbook."

Analysts say Mrs Clinton's tactics are a dramatic change of direction, as she tries to win two crucial primaries on March 4, and stop the Obama bandwagon from getting out of sight after 11 consecutive primaries.

It is a complete reversal from a softer image she portrayed only a few days ago when she ended a nationally televised debate by saying she was "honoured to be here" with Mr Obama in a historic race between a black man and a woman.

In her criticism of her Democratic challenger, she asked: "Since when do Democrats attack one another on universal health care?"

Mr Obama defended the mailings as accurate and rejected Mrs Clinton’s complaint as a political ploy. He said that despite her current criticism of the North American Free Trade Agreement with Canada and Mexico, Mrs Clinton supported it when it passed during her husband’s administration.

"You can’t be for something and take credit for an administration ... and then when you run for president say that you didn’t really mean what you said way back then. It doesn’t work like that," he said to cheers at a rally in Akron, Ohio.

The attacks come as the simmering row over healthcare intensified between the two candidates.

In the last few days, the Clinton camp has criticised two of the Obama campaign’s mailings.

One says that her plan for universal coverage would "force" everyone to purchase insurance even if they cannot afford it. Her plan requires everyone to be covered, but it offers tax credits and other subsidies to make insurance more affordable.

Mr Obama’s plan does not include the so-called "individual mandate" for adults. He has argued that people cannot be required to buy coverage if they cannot afford it. He has said his first priority is bringing down costs.

The Illinois senator’s plan does include a mandate requiring parents to buy health insurance to cover children.

In the overall race for the Democratic nomination, Mr Obama leads with 1,362 delegates. Mrs Clinton has 1,266.5, getting the half-delegate from the Democrats Abroad primary. It will take 2,025 delegates to secure the Democratic nomination at the party’s convention in August.

On the Republican side, John McCain inched closer to clinching the party’s presidential nomination by picking up a total of 18 more delegates yesterday at Republican conventions in American Samoa and the US Commonwealth of the Northern Mariana Islands.

They gave him a total of 976 delegates, well ahead of the former Arkansas Governor Mike Huckabee, who only has 254 delegates overall. It takes 1,191 delegates to secure the Republican nomination at the party’s convention in September.

Meanwhile Ralph Nader, the consumer advocate accused by many Democrats of handing the presidency to President Bush in November 2000, announced today that he would run again.

Mr Nader won more than two per cent of the vote when he stood as the Green Party candidate in 2000, with the election so close that only a small proportion of his support would have put Al Gore, the Democratic candidate, into the White House.

The 73-year-old explained his candidacy by saying that many Americans were disenchanted with the two main parties, but he is thought to have insufficient support to play any major role in the 2008 contest.

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