Romney urged to explain Swiss bank account

Republican donors driving Mercedes and Bentleys have crowded into a series of closed-door Mitt Romney fundraisers in the wealthy Hamptons.

Mitt Romney has privately raised millions of dollars from New York's elite, as Democrats attacked the Republican presidential contender, intensifying calls for him to explain his offshore bank accounts and release several years of tax returns.

The line of attack, dismissed by the Romney campaign as an "unfounded character assault," follows new reports that raise questions about Romney's personal wealth, which could exceed $US250 million ($A243.84 million).

President Barack Obama's re-election campaign is expected to push the strategy throughout the coming week, underscoring their desire to portray Romney as disconnected from the middle-class voters he needs to win the presidency.

"He's the first and only candidate for the president of the United States with a Swiss bank account, with tax shelters, with tax avoidance schemes that involve so many foreign countries," Senator Dick Durbin, an Illinois Democrat, said on CBS' Face the Nation.

He was one of several high-profile Democrats who spoke out on the Sunday morning TV news shows.

Romney may have unintentionally helped the Obama campaign.

Republican donors driving Mercedes, Bentleys - and in one case a candy red 2013 Ferrari Spider - crowded into a series of closed-door Romney fundraisers in the Hamptons, New York's exclusive string of waterfront communities on Long Island's South Shore.

Wall Street bankers and brokerage house chiefs, among others, make the area their weekend playground.

Romney's Hamptons swing follows a weeklong family vacation at his lakeside vacation home in New Hampshire.

Voters are split on whether they trust Romney or Obama more to run the US economy but a majority says that Obama better understands their concerns. The Hamptons crowd, however, saw things differently.

"I think he's a plain talking guy," said Peter Cohen, the former Shearson Lehman Brothers chief who now heads his own investment banking firm, as he chewed a cigar in his black Range Rover outside a Romney fundraiser expected to generate $US3 million.

Romney's day concluded at the Southampton estate of billionaire industrialist David Koch, where donors were asked to give $US50,000 per person or $US75,000 per couple.

Romney is planning a trip overseas ahead of the Republican National Convention in August.

He is set to visit the Olympics in London for the opening ceremonies on July 27 and the beginning of the competition. Then he'll head to Israel, where he plans to meet with Prime Minister Benjamin Netanyahu.

Romney's advisers are considering plans to visit other countries in Europe, though the itinerary is still being finalised.