SOUTHAMPTON, N.Y. — Mitt Romney privately raised millions of dollars from New York's elite on Sunday, as Democrats stepped up calls for him to explain 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 $250 million. President Obama's re-election campaign is expected to push the strategy throughout the coming week, underscoring its 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," Sen. Dick Durbin, D-Ill., said on CBS' "Face the Nation." He's one of several high-profile Democrats who spoke out on the Sunday-morning news shows.

Romney unintentionally may have 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 vacation at his lakeside vacation home in New Hampshire.

Voters are split on whether they trust Romney or Obama more to run the nation's economy, but a majority says that Obama better understands voters' concerns.

The Hamptons crowd, however, saw things differently.

"I think he's a plain-talking guy," Peter Cohen said, referring to Romney. Cohen, the former Shearson Lehman Brothers chief who now heads his own investment-banking firm, made the comment as he chewed a cigar in his black Range Rover outside a Romney fundraiser expected to generate $3 million.

Romney's day concluded at the Southampton estate of billionaire industrialist David Koch, where donors were asked to give $50,000 per person or $75,000 per couple. The event attracted protesters like Robert Shainwald, a 65-year-old retired teacher.

"Romney has no idea what the working person's daily concerns are. How could he?" Shainwald said as he waved a sign offering free vegetables to anyone who wasn't a billionaire.

Romney would be among the nation's richest presidents if elected. He made his fortune at Bain Capital, a Boston-based private equity firm that has become a key part of the argument for his White House bid. He hasn't drawn a regular paycheck in more than a decade, however, and instead has lived off a series of investments.

But Romney has refused to release more than two years of tax returns that would outline those investments, breaking from a precedent set by his father, former Michigan Gov. George Romney, who released 12 years of his tax returns when he sought the presidency a generation ago. And an Associated Press report recently raised questions about a previously undisclosed Bermuda-based company included in Romney's portfolio until the day before he became Massachusetts governor.

Obama adviser Robert Gibbs said Romney easily could clear up questions about his personal finances if he simply released "a series of years" of returns.

"Mitt Romney's father was the pioneer for releasing a series of tax returns," Gibbs said on CNN's "State of the Union." "The best way to figure out if Mitt Romney is complying with American tax law is to have him release more of his tax returns."