WASHINGTON: The US presidential election campaign has intensified as incumbent Barack Obama and his Republican rival Mitt Romney got engaged in sharp exchange of words.

In his speech at Ohio on Tuesday, Romney alleged that Obama is running an election campaign of hate.

"Mr President, take your campaign of division and anger and hate back to Chicago and let us get about rebuilding and reuniting America," he thundered.

The Obama campaign immediately hit back by saying that Romney "seemed unhinged" in his speech.

"Governor Romney's comments tonight seemed unhinged, and particularly strange coming at a time when he's pouring tens of millions of dollars into negative ads that are demonstrably false," said the Obama campaign press secretary, Ben Labolt, in a statement yesterday.

But it all started with the "chains" comment made by US Vice President Joe Biden at an event in Virginia, to which the Romney campaign fought back by alleging that Obama campaign had sunk the presidential election campaign to a new low.

Biden's controversial remarks came while criticising Romney at an election event in Virginia.

"Romney wants to let the -- he said in the first 100 days, he is going to let the big banks once again write their own rules. Unchain Wall Street. They're going to put y'all back in chains," he said.

"He (Romney) said he's going to do nothing about stopping the practice of outsourcing.”

“Republicans even voted down a proposal to say, OK we're not going to give a tax break like we do now to companies who unbolt the machinery on the floor and send it to Singapore," Biden said.

"They get a tax break for that, by the way. We're not going to do that anymore. We're going to give a tax break to any company unbolts their factory stuff in Singapore and brings it back to Danville," said the US Vice President, whose remarks immediately drew strong reaction from the Romney campaign.

"After weeks of slanderous and baseless accusations levelled against Governor Romney, the Obama Campaign has reached a new low," said Romney Campaign National press secretary Andrea Saul.

"President Obama should tell the American people whether he agrees with Joe Biden's comments," Saul demanded.