(CNN) – Barack Obama's campaign took fresh aim at Hillary Clinton Wednesday for refusing to release her tax returns, asking in a memo circulated to reporters, "What does Clinton have to hide?"

"In the face of her unwillingness to release her tax returns, Hillary Clinton has made the false case in this campaign that she is more electable because she has been fully vetted," the memo stated. "When it comes to her personal finances, Senator Clinton’s refusal to release her taxes returns denies the media and the American people the opportunity to even begin that process."

Obama, who released his 2006 tax returns last April, stepped up his pressure on Clinton to release her documents early last month after it was revealed the New York senator loaned her campaign $5 million.