America's presidential campaign this weekend moved from the furious to the near farcical, amid a bitter "dirty tricks" controversy over the authenticity of 30-year-old documents dealing with George Bush's service in the Texas Air National Guard.

America's presidential campaign this weekend moved from the furious to the near farcical, amid a bitter "dirty tricks" controversy over the authenticity of 30-year-old documents dealing with George Bush's service in the Texas Air National Guard.

The rumpus began with a CBS news report on Wednesday evening featuring hitherto unpublished memos purporting to be from Mr Bush's squadron commander of the day. They suggested that the well-connected future president was given special treatment in gaining a coveted slot in the guard in 1968 to avoid being sent to Vietnam, and generally benefitted from political pressure to have his guard performance assessments presented in the best possible light.

But within 24 hours the aged documents were being challenged - raising suspicions that CBS had fallen victim to a hoax by Bush supporters to discredit critics of the President's military record.

Lt Col Jerry Killian, the squadron commander in question, is now dead. But his widow and daughter have said he would never have committed such observations to paper. Meanwhile, arguments rage over whether early 1970s typewriters could have produced the sophisticated word spacing and raised superscript found in the memos.

Yesterday, the Dallas Morning News muddied the waters further, claiming that another officer, said in an August 1973 memo to have asked for the evaluation of Mr Bush to be "sugarcoated", had in fact left the military in March 1972, according to records obtained by the newspaper.

If CBS has been victim of a hoax, it would be a huge embarrassment for the network, its flagship news magazine programme 60 Minutes, and its celebrated anchorman, Dan Rather, who prepared the report. Conservative talk shows are already having a field day, while the White House is quietly gleeful. Yesterday, CBS stuck to its guns, insisting that it had carried out the most thorough checks. "Until someone shows me definitive proof that they are not [authentic], I don't see any reason to carry on a conversation with the professional rumour mill," Mr Rather declared. The public, he added, "is smart enough to see where some of the criticism is coming from". In fact, the controversy is beside the point. Poll after poll has shown that in this election voters are concerned about terrorism, war in Iraq and the economy, rather than events of 35 years ago. John Kerry's highlighting of his 1969 heroics in the Mekong Delta has if anything backfired, provoking a mendacious but highly effective campaign of TV ads by pro-Bush veterans, accusing him of lying over how he obtained some of his medals.

It has, moreover, long been accepted that strings were pulled on Mr Bush's behalf to get him into the guard. Nor has the rumpus had much impact on the wider campaign. According to a new Associated Press poll yesterday, the President holds a 51/46 lead over Mr Kerry among likely voters, and is pulling ahead in key swing states, notably Ohio and Missouri.

The underlying dynamic has changed little since the President gained his "bounce" from the successful Republican convention. Mr Bush holds a wide advantage over Mr Kerry on who can best protect the country from terrorism, the issue which, polls say, matters most to voters. The President has also pulled ahead on the two important "character" ratings of trustworthiness and likeability. In most recent elections, the most "likeable" candidate has won. The folksy, grammar-mangling Mr Bush easily outscores his somewhat stiff Democratic challenger.

"Presidential elections are the most personal vote of all," says William Schneider, a political analyst at CNN and the American Enterprise Institute here. "Americans are choosing the guy who's going to be on the TV screen in their living room for the next four years. They want to be able to like him."