More than 100 days after Barack Obama clinched the Democratic presidential nomination, Bill Clinton was finally due to share a stage last night with the man who proved to be his wife’s nemesis.

The former President, who for months has vacillated between rage, envy, petulance, open scorn and – only lately – solid support when the topic has turned to Mr Obama, at last agreed to campaign with him in the critical battleground of Florida.

The backing from Mr Clinton, just six days before the election, came as John McCain unleashed a new and incendiary line of attack against Mr Obama, linking his rival to the cause of Palestinian terrorism as he sought to tap in to the Jewish vote in Florida. New polls show that the race there is too close to call.

Mr McCain accused theLos Angeles Timesof deliberately suppressing a videotape that it has obtained of a 2003 banquet in Chicago during which Mr Obama praised Rashid Khalidi, a Palestinian scholar. In the 1970s Mr Khalidi was a spokesman for the Palestine Liberation Organisation when it was a designated terrorist group.

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The newspaper wrote a detailed account of the dinner in April, as part of a story about Mr Obama’s ties with Palestinians and Jews as he navigated Chicago politics. The Democrat made his speech at a farewell dinner for Mr Khalidi, who was a friend and colleague of Mr Obama at the University of Chicago. The newspaper said that it was refusing to release the tape because it was provided by a confidential source on condition that it was never broadcast.

“I’m not in the business of talking about media bias, but what if there was a tape with John McCain with a neo-Nazi outfit being held by some media outlet?” Mr McCain said during a radio interview in Miami. “I think the treatment of the issue would be slightly different.” Mr Obama, without referring specifically to Mr McCain’s comments about Mr Khalidi, said at a rally in North Carolina that he was “sorry to see my opponent sink so low”. He added: “By the end of the week, he’ll be accusing me of being a secret communist because I shared my toys in kindergarten.”

It emerged that Mr McCain had his own connection to Mr Khalidi. The Arizona senator is chairman of the International Republican Institute, which donated nearly $500,000 (£300,000) in 1993 to a Palestinian research group founded by Mr Khalidi.

In the final week of campaigning the candidates have unleashed a barrage of advertisements, mostly in a string of previously “red” Republican states that Mr McCain is struggling to defend. New polls yesterday showed Mr Obama with leads in Ohio, Virginia, Colorado and New Mexico, with two surveys appearing to show that Mr McCain was in danger of losing his home state of Arizona.

Yet the polls remain highly volatile. In national surveys Mr Obama’s lead is anything from two points to fifteen, with Mr McCain’s aides insisting that the race is far closer than it appears. They maintain that the country could be heading for a repeat of 1948, when Harry Truman pulled off an improbable victory against a young, eloquent and overconfident Thomas Dewey.

Bill McInturff, Mr McCain’s pollster, claimed in a memo that 8 per cent of voters in the battleground states were undecided, and that most would break late for the Republican. During the Democratic primary battle many undecided voters shifted behind Mrs Clinton at the last minute.

Last night the Illinois senator was due to run a 30-minute prime-time “in-formercial” on four networks. The broadcast, costing more than $3 million, was the ultimate sign of Mr Obama’s spending advantage over his rival. The McCain camp said that it smacked of grandiosity and excess.

For the first time the Obama campaign also released an advertisement criticising Sarah Palin, Mr McCain’s running-mate, amid polls showing that she has become a drag on the Republican ticket. Flagging up a previous quote from Mr McCain – “I might have to rely on a vice-president that I select” for expertise on economic issues – the commercial ends with “His choice?”, followed by foot-age of a winking Alaska Governor.

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