The campaign video which attacks John McCain over his links to Charles Keating
US Democratic presidential candidate Barack Obama has attacked John McCain's links to a 1980s financial scandal.
He also accused his Republican rival of being more focused on running a smear campaign than on fixing the US economy.
It comes after Mr McCain's running mate Sarah Palin accused Mr Obama over the weekend of associating with terrorists.
Mr Obama once served on a charity board with an ex-member of US-based militant group Weather Underground, Bill Ayers, now a University of Illinois professor.
He has denounced Mr Ayers' radical past.
Mr McCain spent the weekend in Arizona preparing for Tuesday's presidential debate but Mrs Palin, governor of Alaska, held several rallies.
She told supporters in Colorado and California that the time had come to take "the gloves off" - suggesting that Mr Obama's character may be the subject of further attacks.
In a new internet video being e-mailed to supporters, the Obama campaign launched its own attack on Mr McCain over his connections to tycoon Charles Keating, who was convicted of securities fraud after his savings and loan scheme collapsed.
Senator McCain and his operatives are gambling that he can distract you with smears rather than talk to you about substance
Mr McCain was one of five senators - known as the Keating Five - to be investigated by a Senate ethics panel over their intervention with banking regulators on behalf of Keating.
He was found to be less involved with Keating than the other senators but was criticised for "poor judgement".
Mr McCain has himself described the affair as "the worst mistake of my life", and one which led him to sponsor legislation on campaign finance reform.
In an email to supporters, Obama campaign manager David Plouffe said: "The McCain campaign has tried to avoid talking about the scandal, but with so many parallels to the current crisis, McCain's Keating history is relevant and voters deserve to know the facts - and see for themselves the pattern of poor judgement by John McCain.''
The Obama campaign has set up a special website where viewers can see the short internet video and a 13-minute documentary on Mr McCain's involvement in the Keating scandal.
Speaking at a rally in North Carolina on Sunday, Mr Obama also questioned Mr McCain's campaign tactics, suggesting he was seeking to turn the focus away from the current economic crisis.
"Senator McCain and his operatives are gambling that he can distract you with smears rather than talk to you about substance," he said.
"They'd rather try to tear our campaign down than lift this country up. It's what you do when you're out of touch, out of ideas and running out of time."
That message is echoed in a new campaign advert, to be broadcast on cable TV from Monday, which seeks to portray Mr McCain's response to the financial crisis as fumbling and unreliable.
The commentary says: "Erratic in a crisis. Out of touch on the economy."
Meanwhile, singers Bruce Springsteen and Jay-Z took part in concerts over the weekend, organised by the Obama campaign, aimed at encouraging new voters to register.
Monday is the deadline for voter registration in more than a dozen states, including Pennsylvania, Virginia, Michigan, Ohio, Indiana and Florida.
Voter turnout could be vital in deciding the outcome of the 4 November presidential election.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.