TOP Democrats have launched a coordinated assault on Mitt Romney's use of offshore tax havens, demanding the Republican release more tax returns ahead of the 2012 presidential election.

Robert Gibbs, a senior campaign adviser to President Barack Obama, delivered the pithiest barb on CNN, saying: "This is a guy whose slogan is 'believe in America' when it should be 'business in Bermuda.'"

Over on Fox News Sunday, Democratic Party chairwoman Debbie Wasserman Schultz said: "Americans need to ask themselves, why does an American businessman need a Swiss bank account and secretive investments like that?"

Maryland Governor Martin O'Malley, senior Senator Dick Durbin and other Obama campaign aides also lashed out to portray Romney as a "secretive" businessman who was hiding much of his money offshore to avoid the taxman.

The Republican presidential hopeful's wealth, estimated to be around $250 million, has repeatedly emerged as an issue during the campaign as Obama tries to paint him as out of touch with ordinary Americans before the November vote.

Romney's tax rate has been a particular bone of contention, as have questions about the manner in which he acquired his fortune at the helm of the Boston-based investment firm he founded, Bain Capital.

Under pressure from Republican primary opponents, he reluctantly released his return for 2010 and an estimate of his taxes for 2011 in January, but has yet to do the same for previous years, including for when he worked at Bain.

Blistering attack ads accusing Romney of shipping US jobs overseas and breaking up companies for profit during his time at Bain are credited with giving Obama an edge in crucial rust belt swing states Pennsylvania and Ohio.

A Vanity Fair report less than a week ago shifted the focus back onto Romney's millions tucked away in Switzerland and in offshore accounts in the Cayman Islands and Bermuda.

Obama aides and senior Democrats took to the Sunday morning talk shows en masse to criticize Romney on that issue, hoping to reframe the campaign after a week in which more damaging jobs numbers hurt the president's chances.

"I'd really like to see Mitt Romney release more than one year of tax records because there's been disturbing reports recently that he's got a secretive Bermuda corporation that no one knows anything about, investments in the Caymans, (some) kind of Swiss bank account,'' Mrs Wasserman Schultz said.

Vanity Fair reported on a corporation established in the 1990s in Bermuda and holdings in the Cayman Islands.

"Did he put his money there to avoid paying US taxes? We don't know, because he won't follow precedent and release his tax returns," Ben LaBolt, a spokesman for the Obama campaign, says in a new online video.

"Mitt Romney could be the first president in history to stash millions offshore but so far he has failed to answer simple questions that might explain why, and he refuses to release the documents that might answer those questions."

The video also took Romney to task over the Bermudan corporation, asking why he transferred ownership to a blind trust in his wife's name the day before being sworn in as governor of Massachusetts.

"What taxes would Romney have paid if his money was invested here in America?" it asks.

Romney has amassed vast wealth since founding Bain Capital in 1984, and he consistently says his successful business experience is what puts him in better position than Obama for turning around the sluggish US economy.

Much of Romney's fortune lies well hidden in a network of opaque offshore investments including some $30 million in the Cayman Islands, Vanity Fair reported Tuesday.

Romney continues to have personal interests in at least 12 of the 138 funds organized by Bain in the Caymans, where such investments are hidden behind confidentiality disclaimers, making an assessment of his true wealth virtually impossible, the magazine said.

He also holds a Swiss bank account - with $3 million in it, according to 2010 tax returns - and other interests in tax havens such as Bermuda, Vanity Fair said in its August issue.

The Romney campaign hit back at the video and the new attacks, with spokeswoman Andrea Saul saying in a statement: "The Obama campaign's latest unfounded character assault on Mitt Romney is unseemly and disgusting.

"Mitt Romney had a successful career in the private sector, pays every dime of taxes he owes, has given generously to charitable organizations, and served numerous causes greater than himself," it said.

"Barack Obama has become what he once ran against -- a typical politician willing to use false and dishonest attacks to save his job after failing to do his job. The American people expected more from this president, and he continues to let them down."