The Obamas' joint income has risen dramatically since 2005
Democratic presidential hopeful Barack Obama has published seven years of tax returns on his website and urged his rival, Hillary Clinton, to do the same.
Mr Obama's spokesman said he hoped the release of tax returns would encourage her to let people "see her finances" before the key primary in Pennsylvania.
Mrs Clinton said she hoped to release her tax returns "within the next week".
Earlier, Mrs Clinton was forced to explain how she came to misrepresent a visit she made to Bosnia in the 1990s.
She said last week that she and her daughter, Chelsea, "ran with our heads down" to avoid sniper fire when she arrived at Tuzla airport in 1996.
But archive television footage broadcast on Monday showed her smiling as they were greeted by a Bosnian government welcoming committee.
Mr Obama's aides said the admission was proof that Mrs Clinton had overstated her foreign policy experience.
'Commitment to transparency'
Later, the Illinois senator released copies of his tax returns covering 2000 to 2006, which showed his income had risen dramatically with the publication of two books.
Mr Obama and his wife Michelle's joint income jumped from $275,000 (£137,000) in 2004 to $1.6m (£0.8m) in 2005, when he was paid $1.2m (£0.6m) for the re-release of his first book, Dreams From My Father.
The publication of his second book, The Audacity of Hope, in 2006 earned him more than $500,000 (£250,000), half of their joint income that year.
"Releasing tax returns is a matter of routine," Mr Obama's spokesman, Robert Gibbs, told reporters.
"We believe the Clinton campaign should meet that routine standard and meet that routine standard now."
"This campaign is now going on into its 14th month and I don't think voters should have to wait until three days before the next primary to learn more about the finances of the Clintons."
Mrs Clinton, campaigning in Pennsylvania ahead of the primary election on 22 April, said she hoped her tax returns would be released "within the next week".
Her campaign also issued a statement calling Mr Obama's personal finances "opaque" by comparison with her family, who released 20 years of tax returns whilst her husband, Bill, was in the White House.
Mr Gibbs dismissed the claim and insisted on his candidate's "commitment to transparency and open government", pointing out that the Clintons have not released any tax returns since Mr Clinton's presidency ended in 2001.
Earlier, Mrs Clinton was forced to admit her description of "landing under sniper fire" while visiting Bosnia in 1996 was wrong.
An aide to Mrs Clinton said she "misspoke" about the Bosnia visit
"So I made a mistake," she said while visiting Greensburg.
"That happens. It proves I'm human which, you know, for some people, is a revelation."
The Obama campaign said the story "joins a growing list of instances in which Senator Clinton has exaggerated her role in foreign and domestic policy-making".
Mrs Clinton tried to change the subject by revisiting last week's controversy over fiery sermons given by Mr Obama's former pastor, the Reverend Jeremiah Wright, telling a newspaper that she would have left Mr Wright's church.
"He would not have been my pastor," she told the Pittsburgh Tribune-Review.
"You don't choose your family, but you choose what church you want to attend."
Mr Obama gave a major speech on race relations last week in response to the furore over Mr Wright's remarks but, while condemning what he said, refused to "disown" him.
Mr Obama is ahead of Mrs Clinton in terms of the number of delegates won in the Democratic primary elections so far.
The delegates will choose in August which candidate is to be the party's nominee in November's presidential election, standing against the presumptive Republican nominee, Senator John McCain.