Hillary Clinton accused Barack Obama yesterday of being nothing more than an old-school candidate who had swapped his message of hope for dirty politics, as the Democratic rivals unleashed their most personal attacks yet on the eve of today’s Pennsylvania primary.

After six bruising weeks since their last contest, a period in which Mrs Clinton was caught telling a false story about landing in Bosnia under sniper fire and Mr Obama has suffered controversy involving his former pastor and comments he made about working-class voters, both rivals accused each other of negative, dishonest and demeaning tactics.

The intensity and tone of the attacks in the final hours of campaigning reflect not only the high stakes in Pennsylvania but also the extent to which the state’s long battle has forced Mr Obama to engage in the type of political mudwrestling he decried as his candidacy and message of hope and renewal took off last year.

In the past 48 hours Mr Obama has questioned Mrs Clinton’s honesty and trustworthiness, accused her of “saying whatever folks want to hear”, told a crowd that she “twisted the knife” in their debate last week, and declared to another that she “would be vastly different than George Bush would be – but that’s a very low bar”.

Exploiting his huge financial advantage, Mr Obama has smashed primary spending records in Pennsylvania, paying out about $9 million (£4.5million) on television commercials to Mrs Clinton’s $4 million, including one alleging that the former First Lady is a Washington crony in hock to lobbyists and special interest groups.

At an event in Blue Bell, outside Philadelphia, Mr Obama nevertheless sought to stick to his message of change, saying: “We’ve got to get out of the typical pattern of politics.”

Mrs Clinton, for whom a good win in Pennsylvania is critical to her central argument that she is more electable than Mr Obama, said that her rival was now undermining his message. She called his tactics very reminiscent of the “old politics” that he has criticised, and said that her rival had started throwing the kitchen sink at her, a strategy her aides said they deployed against Mr Obama before March’s contests in Texas and Ohio.

She also launched her own negative advertisements in which, not for the first time, she used the threat of another terrorist attack to try to stoke fears among voters about her rival’s lack of experience.

The 30-second commercial, aired across Pennsylvania, included imagery of Osama bin Laden, the Pearl Harbor attacks, President Kennedy and the Cuban missile crisis. While not directly referring to Mr Obama, it closed with the question: “Who do you think has what it takes?”

In response, Mr Obama’s aides accused Mrs Clinton of resorting to President Bush’s “politics of fear”. Bill Burton, a spokesman for the Illinois Senator, said: “It’s ironic that she would borrow the President’s tactics in her own campaign and invoke bin Laden to score political points. We already have a president who plays these politics of fear, and we don’t need another.”

In a reference to their prime-time debate last week, in which Mr Obama was forced to defend his patriotism, the incendiary remarks of his former pastor and his ties to a former violent radical of the 1960s, the Clinton campaign launched a second commercial, which stated: “There are more and more questions about Barack Obama. Instead of attacking, maybe he should answer them.”

Mrs Clinton has a steep uphill battle to wrench the nomination from Mr Obama’s grasp. With ten contests left, he has an insurmountable lead among elected delegates and leads Mrs Clinton among the popular vote. She is also mired in debt. Mr Obama, who has raised $237 million, had $42 million ready to spend as of April 1, and little debt. Mrs Clinton began the month $10.3 million in debt, and with only $9 million in the bank.

Her only hope of defeating Mr Obama is to persuade a significant majority of uncommitted “super-delegates”, the Democratic Party’s elected politicians and senior officials, who can back either candidate, to overturn the will of the primary voters and hand her the nomination.

She is arguing forcefully that Mr Obama is an untested presidential election risk and that she has far more chance of beating John McCain in November.

A victory today for Mr Obama would all but end her campaign. But that is unlikely, as he readily conceded yesterday. “I’m not predicting a win,” Mr Obama told Pittsburgh’s KDKA radio station. “I’m predicting it’s going to be close and that we are going to do a lot better than people expect.”

A big win for Mrs Clinton, especially ten points or more, would keep her candidacy alive and send the marathon nomination battle on to Indiana and North Carolina, which vote on May 6.

New polls released yesterday showed Mrs Clinton 7 to 10 per cent ahead in Pennsylvania, although she was 16 points ahead just a few weeks ago. The surveys again showed Mr Obama trailing badly among whites, blue-collar voters and women, key demographic groups that he will have to attract in large numbers to defeat Mr McCain. Repeating the trend of previous contests, Mr Obama leads overwhelmingly only among Pennsylvania’s black and young voters.

The length and bitterness of the campaign is alarming many Democratic strategists. In head-to-head match-ups, Mr McCain is tied with Mrs Clinton and Mr Obama, and reported his best monthly fundraising total of $15.2 million, raised in March.

In the Democratic debate last week, one issue Mr Obama faced was his visit in 1995 to the home in Chicago of William Ayers, a former member of a Vietnam-era extremist group, the Weather Underground, which bombed the Pentagon and other government buildings. Mr Ayers, now a professor of education in Chicago, was introducing Mr Obama to Democratic figures in the city during his first run for the Illinois state senate.

In an interview on Sunday, Mr McCain, unprompted, raised the issue, saying Mr Obama’s links to Mr Ayers were open to question. It was a clear sign of how Republicans are going to attack the Illinois Senator if he becomes the nominee.

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