Republican presidential hopeful John McCain has ratcheted up his assault on his Democratic rival's character, with a new advert branding him "too risky".
The ad accuses Barack Obama of hiding ties to Bill Ayers, an ex-member of former militant group, the Weathermen.
Mr Obama has insisted he is not close to Mr Ayers. A spokesman said the ad was a "desperate and dishonest" attack.
Meanwhile, in a bid to attract young voters, Mr Obama has reportedly bought advertising time on three MTV networks.
Officials at MTV told the New York Times newspaper that his campaign had bought advertising time on Comedy Central, VH1 and Spike and could start running ads very soon.
Mr Obama is the first US presidential candidate to buy advertising time on the networks, which did not previously allow political advertising, an MTV spokesman said.
The Illinois senator holds a widening lead over Mr McCain in polls conducted nationwide and in a number of key states.
Mr Obama began a two-day bus tour of Ohio on Thursday, while his running mate Senator Joe Biden headed to Missouri.
Speaking at a rally in Dayton, Ohio, Mr Obama said Mr McCain's plan for the government to buy up loans from troubled homeowners would cost US taxpayers billions of dollars and reward the lenders "who got us into this mess".
Mr McCain and running mate Sarah Palin, governor of Alaska, were to campaign together in Wisconsin.
'Judgement and candour'
Mr McCain's latest video ad, launched online, continues a line of attack begun by Mrs Palin over the weekend, when she said Mr Obama had been "palling around" with a terrorist.
The Weathermen, or Weather Underground, waged a violent campaign against the Vietnam War during the 1960s and 1970s, when Mr Obama was a child.
Mr Obama once served on a charity board with Mr Ayers.
The advert describes Mr Obama and Mr Ayers as "friends" who have "worked together for years "and claims that Mr Obama's political career was launched in Mr Ayers' living room.
It also questions Mr Obama's "judgement and candour" when questioned about the relationship now.
"Americans say, 'Where's the truth, Barack?'" the ad voiceover says. "Barack Obama. Too risky for America."
Obama campaign spokesman Robert Gibbs, speaking on ABC News, said the advert was "meant to cover up that John McCain has virtually no answers for the economic crisis".
Mr Obama has repeatedly denied close ties to Mr Ayers, now a university professor in Chicago - a claim supported by independent observers, the AFP reports.
"The notion that somehow he has been involved in my campaign, that he is an adviser of mine, that he - I've been 'palling around with a terrorist', all these statements are made simply to try to score cheap political points," he said on Wednesday.
(BBC)
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