(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."
David Axelrod, the Obama campaign's chief strategist, also said in a conference call with reporters Wednesday that Clinton has a history of non-disclosure and is the least-vetted candidate in the presidential race.
Clinton had said she will only release her tax returns if she captures the Democratic nomination, though her communications director, Howard Wolfson, issued a statement Wednesday indicating she would do so around April 15.
"Over 20 years of the Clintons' tax returns are in the public domain," he said. "Their tax returns since they left the White House will be made available on or around April 15. This information will be in addition to 15 years of uninterrupted public financial disclosure reports."
"Instead of making false attacks, we urge Senator Obama to release all relevant financial and other information related to indicted political fixer Tony Rezko," Wolfson also said.
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.
"I think the American people deserve to know where you get your income from," he said. "I've disclosed my income tax returns. I think we set the bar in terms of transparency and disclosure that has been a consistent theme of my campaign and my career in politics.”
The New York senator did file a Senate disclosure form last May that revealed the Clinton's income for the previous year was upwards of $12 million. But, according to the Wall Street Journal, a tax return would present a more comprehensive account of the Clinton's income, and include revenue sources that aren't required on Senate disclosure forms.
"Though her campaign has tried to kick the issue down the road, Democratic voters deserve to know, right now, why it is she is hiding the information in her tax returns from last year," the Obama memo stated. "The Clinton campaign has said that they have released copious amounts of financial information but there are many questions about their private dealings that could be answered in their tax returns but not in the information that is currently available.
- CNN Ticker Producer Alexander Mooney