US Republican presidential candidate 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 an ex-member of former militant group, the Weathermen, Bill Ayers.
Senator Obama has insisted he is not close to Mr Ayers. A spokesman said the ad was a "desperate and dishonest" attack.
The Illinois senator holds a widening lead over Senator McCain in polls conducted nationwide and in a number of key states.
Senator McCain's latest video ad, launched online, continues a line of attack begun by vice presidential candidate Sarah Palin over the weekend, when she said Senator 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.
Senator Obama once served on a charity board with Mr Ayers.
The advert describes them as "friends" who have "worked together for years "and claims that Senator Obama's political career was launched in Mr Ayers' living room.
It also questions Senator 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."
An Obama campaign spokesman told ABC News the advert was "meant to cover up that John McCain has virtually no answers for the economic crisis".
AFP reports Senator Obama has repeatedly denied close ties to Mr Ayers, now a university professor in Chicago - a claim supported by independent observers.
Meanwhile, in a bid to attract young voters, Senator Obama has reportedly bought advertising time on three MTV networks.
Officials at MTV told The New York Times that the campaign has bought advertising time on Comedy Central, VH1 and Spike and could start running ads very soon.