The two campaigns clashed over the economy after Thursday's announcement by the US commerce department that the nation's economy shrank by 0.3 per cent in the last financial quarter - its worst showing since 2001.

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At a rally in Sarasota, Florida, Obama said the news reflected the failed Republican policies of McCain and George Bush, the US president.

"If you want to know where John McCain will drive this economy, just look in the rear-view mirror," he said, continuing the theme of a new television advertisement launched by his campaign.

McCain jumped on the record quarterly profits posted by Exxon Mobil, noting Obama had supported tax breaks for the oil industry in a 2005 energy bill.

"Senator Obama voted for billions in corporate giveaways to the oil companies. I voted against it," McCain told supporters in Defiance, Ohio.

McCain later campaigned alongside Joe "the Plumber" Wurzelbacher, the Ohio plumber who clashed with Obama over his tax policies earlier in the campaign, in the town of Mentor.

No US president has been elected without winning Ohio since 1960, and McCain trails Obama in state opinion polls.

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The Obama campaign aired a $4m, 30-minute national television advertisement in a bid to sway voters, focusing on the economy and healthcare.

The Illinois senator told ABC television on Thursday he ran the advertisement because "when the polls close on Tuesday, you don't want to say to yourself, 'here's something I didn't do, here's an argument I didn't make, here's a hand I didn't shake'."

However McCain dismissed the advertisement during a CNN interview on Wednesday as a "gauzy, feel-good commercial".

More than 33.5 million viewers tuned in to watch the message, which aired on three major broadcast networks and four smaller channels, Nielsen Media Research reported on Thursday.

Al Jazeera's Rosiland Jordan in Ohio said McCain had returned to a populist message based on Ronald Reagan's mantra of low taxes that had proved successful in the 1980s.

But Jordan said questions remained over why McCain was targeting Ohio, which has 20 electoral college votes, so heavily when more college votes - 27 -  were at stake in another battleground state - Florida.

"This may be a question of money - something that the campaign does not want to discuss."

New polls have indicated Obama maintains a strong lead over his Republican rival in four previously Republican states and is tied in two others ahead of the vote on November 4.