Mr McCain accused Mr Obama getting a felon to help him buy his home
Democratic presidential hopeful Barack Obama has hit out at opponent John McCain for being unable to tell an interviewer how many homes he owns.
Mr McCain's staff later said that the Republican contender owned "at least four" houses, although Mr Obama claimed that Mr McCain actually owned seven.
The row started when a video was posted on the internet accusing Mr McCain of enjoying a lavish lifestyle.
The McCain camp said a convicted felon had helped Mr Obama to buy his home.
The BBC's Katty Kay followed the makers of the video as it was being produced.
During an interview with reporters from the Politico website, Mr McCain was asked how many houses he owned:
"I think - I'll have my staff get to you," he said.
"It's condominiums where - I'll have them get to you."
Seizing on Mr McCain's remarks, Mr Obama's campaign released an attack advert saying that if "Mr McCain has lost track of how many houses he owns... here's one house America can't afford to let John McCain into" over footage of the White House.
In a reference to recent positive remarks by Mr McCain about economic conditions, Mr Obama said in a speech to supporters: "If you don't know how many houses you have, then it's not surprising that you might think the economy is fundamentally strong."
The BBC's Katty Kay talks to the makers of the anti-McCain video
Mr McCain's camp responded to the Obama campaign's criticisms by releasing its own advert, accusing Mr Obama of "getting help" from convicted felon Tony Rezko to buy his million-dollar mansion in Chicago.
The independent internet video that triggered the row was produced by Robert Greenwald, an Obama supporter who has put together a number of web videos in support of the Democratic hopeful.
BBC correspondent Katty Kay spoke to Mr Greenwald as he was making the video, before the furore erupted.
He told the BBC that he spotted early on the potential of the internet as a means of getting across a political message.
"We said: 'Let's see if we can use the new technology, not where it had been primarily used - which was naked women falling down in showers and other high cultural events like that - but we could use it for political storytelling'. And we've just passed 23 million views on our videos."
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