Republican presidential candidate John McCain stepped up his attacks on Democratic opponent Barack Obama in a new advert Friday criticising his association with a former 1960s radical. Obama fired back, accusing McCain of trying to divide the country.

Republican presidential candidate John McCain stepped up his attacks on Democratic opponent Barack Obama in a new advert Friday criticising his association with a former 1960s radical. Obama fired back, accusing McCain of trying to divide the country.

With less than four weeks to go before election day on November 4, McCain has increased personal attacks against his rival as his poll numbers have fallen amid voter alarm over the fate of the economy, seen as McCain's weak spot.

The new commercial focuses on Obama's past association with William Ayers, a founder of the violent Vietnam-era group the Weather Underground.

News organizations have reported that Obama and Ayers are not close, but worked together on two non-profit organisations from the mid-1990s to 2002. Ayers, now a college professor, also hosted a small meet-the-candidate event for Obama in 1995 as he first ran for the state senate.

The advertisement says: "When convenient, he worked with terrorist Bill Ayers. When discovered, he lied."

The commercial is arguably McCain's sharpest yet and uses Obama's link to Ayers to assert that Obama has "blind ambition" and "bad judgment".

Obama, who was a child when Ayers's group committed acts of domestic terrorism, has denounced Ayers's radical views and actions. He also said this week that he thought Ayers had been rehabilitated.

The Democrat did not respond directly yesterday to the claim that he associated with a former terrorist. Instead, he told Ohio voters: "It's not hard to rile up a crowd by stoking anger and division."

Obama added that Americans were not looking for someone who can divide the country, rather "someone who can lead this country Now more than ever it is time to put country ahead of politics."

The new advert, which the campaign says will run nationally, comes as the Republican National Committee runs its own TV commercial in the battleground states of Indiana and Wisconsin that also seeks to sow doubts about Obama's political upbringing. It links Obama to Ayers and other figures in what it calls the "shady politics" of Chicago.

The attacks end a week in which the McCain campaign deployed vice-presidential nominee Sarah Palin to question Obama's character. Last weekend, she charged that Obama had been "palling around with terrorists".

Obama, who would be the first black president, has said his opponent was highlighting the Ayers link to "score cheap political points".

McCain has been struggling to gain ground in the polls. Obama has a sizeable lead in the Gallup Poll daily tracking survey, 51% to 41%. Several polling organisations now put McCain even or behind in must-win states captured by George W Bush in 2000 and 2004.

Obama's cause has also been helped by his hefty advertising budget. His campaign announced he had planned a half-hour advert for prime-time national television on October 29, six days before election day.

Meanwhile, the McCain campaign sought to pre-empt what could be an embarrassing ethics report due out yesterday on Palin into allegations that she abused her power as Alaska governor by sacking the state's public safety commissioner.

The commissioner, Walter Monegan, says he was dismissed in July for resisting pressure from Palin's husband, Todd Palin, and numerous top aides to sack Alaska state police officer Mike Wooten, Palin's former brother-in-law.

The McCain-Palin camp released its own report written by campaign staff, which they said relied on public filings and an affidavit that Todd Palin submitted to politicians investigating the issue, and clearing Palin.

"The following document will prove Walt Monegan's dismissal was a result of his insubordination and budgetary clashes with Governor Palin and her administration," campaign officials said.-AP