March 26 (Bloomberg) -- Democratic presidential candidate Barack Obama put rival Hillary Clinton on the spot by releasing his full tax returns and challenging her to do the same.

By disclosing his 2000-2006 returns earlier than is customary, the Illinois senator is forcing Clinton either to reveal details about investments by her husband, former President Bill Clinton, or to face more questions about what they aren't making public.

Obama's action ``makes a statement,'' said Joe Thorndike, who tracks presidential-tax returns at Tax Analysts, a Falls Church, Virginia-based publisher. ``They've clearly drawn some battle lines here with the Clinton campaign. Obama is looking for an edge in the openness front here, and I think he's successfully claimed it.''

The disclosure will also increase pressure on Arizona Senator John McCain, 71, the presumed Republican nominee, to release his returns, Thorndike said. ``If there's some moral burden'' on the Clintons to release their returns, ``there is at least as much and probably more of a burden on McCain to do the same,'' he said. Campaign adviser Steve Schmidt said McCain will release his tax returns in mid-April.

Meeting His Standard

Clinton, 60, a New York senator, said yesterday that she would offer additional information on her finances ``within the next week,'' although she didn't say whether that would include full returns, or only a summary.

When she does, ``she'll have 25 years of tax information available as well as 15 years of personal financial-disclosure forms,'' Clinton spokesman Phil Singer said, referring to filings dating back to when her husband was governor of Arkansas. The Clintons haven't released any tax forms since their 1999 filing.

Obama's campaign called on Clinton to meet his standard of disclosure. Obama, 46, initially released his 2006 tax returns a year ago.

He wants to turn ``the page on secrecy to open government'' said Robert Gibbs, Obama's communications director. ``If Senator Clinton shares those same ideals, all she has to do is send someone down to Kinko's to photocopy tax returns and post them immediately on their Web site.''

Presidential candidates aren't required to release tax returns. Most have chosen to do so, although usually not until they have secured the nomination. President George W. Bush didn't release his 1999 return until less than a month before the 2000 election, yet he had done so for earlier years while governor of Texas.

Kerry's Finances

While Massachusetts Senator John Kerry released his returns in December 2003, before he clinched the Democratic nomination, his wife, Teresa Heinz Kerry, heir to a $500 million fortune, initially refused to do so. She eventually made them public three weeks before the election.

The Clintons' tax returns might shed light on Bill Clinton's financial ties to Vinod Gupta, a longtime supporter, and California billionaire Ron Burkle.

While Hillary Clinton's Senate disclosures report only that her husband has earned more than $1,000 from each relationship, the New York Times reported in May that Gupta's data-mining company, infoUSA Inc., has paid Clinton more than $2 million in consulting fees and spent $900,000 to fly him around the world.

Meanwhile, the former president stands to make tens of millions of dollars with little risk if investments with Burkle's Yucaipa Cos. profit beyond a certain level, the New York Times reported in 2006.

Minimizing Taxes

Bill Clinton has a financial stake in three investment entities registered in the Cayman Islands by Yucaipa. Investing in a Cayman Islands partnership is a legal strategy used to minimize taxes because the country has no taxes of its own. The Clintons would still owe U.S. taxes on any income earned there.

Gibbs said the couple should reveal how the former president is compensated by Yucaipa -- whether at a salary taxable at rates as high as 35 percent, or with equity, which could be as low as 15 percent.

Jay Carson, a Clinton spokesman, said in December the Clintons pay what they owe in taxes on income from the funds, although he wouldn't elaborate on the rate they pay.

Obama's returns showed that he and his wife, Michelle, donated less than 1 percent of the $1.2 million they earned from 2000 through 2004 to charities.

As Generous as Possible

Bill Burton, a campaign spokesman, said the Obamas gave as much as they could afford. ``As new parents who were paying off their large student loans, giving $10,000 to charity was as generous as they could be at the time,'' he said.

They increased their giving as their income rose after he published a bestselling book. In 2005 and 2006, they donated $137,622 on a combined income of $2.6 million, or more than 5 percent. Though they have yet to make their 2007 returns public, their donations totaled $240,000, Burton said.

The Obamas' tax returns also show they had little savings in taxable accounts during the same period. They reported no dividends or capital gains and reported a total of $33 in taxable interest, all of which was received in 2002.

To contact the reporters on this story: Julianna Goldman in Washington at jgoldman6@bloomberg.net; Ryan J. Donmoyer in Washington at rdonmoyer@bloomberg.net

Last Updated: March 26, 2008 10:16 EDT