SOUTHAMPTON, N.Y. -- Mitt Romney privately raised millions of dollars from New York's elite Sunday as Democrats launched coordinated attacks against the likely Republican presidential nominee, intensifying 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 Barack Obama's re-election campaign is expected to push the strategy throughout the week, portraying 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.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 isn't a billionaire.Romney has refused to release more than two years of tax returns, 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.Questions have arisen about a previously undisclosed Bermuda-based company included in Romney's portfolio until the day before he became Massachusetts governor.