Former Sen. John Edwards, D-N.C.,worked for a hedge fund while heading a poverty center in between his presidential campaigns. But since he isn't telling, voters can't know how much money he earned.
Former Gov. Mitt Romney, R-Mass., says his wife, Ann, once donated to Planned Parenthood, but that he never contributed to an abortion-rights group himself. But there's no way for the media and the public to check that claim.
In a break with the tradition of recent presidential campaigns, most of the major presidential candidates aren't releasing their income-tax filings.
Edwards has indicated that he will keep his tax returns private, and while Romney is still considering his options, he has never released his returns in previous runs for office.
Sens. John McCain, R-Ariz., and Hillary Rodham Clinton, D-N.Y., aren't saying whether they will or not, but neither has released income tax forms filed this year.
That means voters are likely to know less about the income sources, personal wealth and charitable inclinations of the presidential candidates than in any election in the past generation.
"When you run for president, you really have to open yourself up to the American people," said Mary Boyle, a spokeswoman for Common Cause, a government watchdog group. "If you're asking voters of this country to elect you as president, it's reasonable and rational that your tax returns are made public."
The release of candidates' tax forms has become common practice in presidential campaigns since the Watergate era of the early '70s.
Since 1984, only one major-party presidential candidate -- Bill Clinton in 1992 -- has refused to release the tax forms he sent to the Internal Revenue Service.
In 1996, Clinton did release his forms, and Republican nominee Bob Dole released his tax returns going back 30 years.
Candidates, including 2004 Democratic presidential nominee John Kerry in 2004, and Democratic vice presidential candidate Geraldine Ferraro in 1984, were criticized for not releasing their spouses' returns but offered no resistance to releasing their own.
Yet as the 2008 election draws near, the only top-tier candidate who has committed to releasing his 1040 forms is Sen. Barack Obama, D-Ill., who already made public the return he filed this year.
Rudolph Giuliani, R-N.Y., released his while he served as New York City's mayor but has not said whether he will revive the practice during his presidential run. (The tax forms from his mayoralty presented an unwelcome distraction to his campaign this week, when reporters tallied up six separate donations he and his then-wife made in the 1990s to Planned Parenthood, a prominent abortion-rights group.)
The candidates who keep their returns private generally note they are complying with all federal regulations with regard to financial disclosures.
The candidates are required to submit standard financial disclosure forms -- due next week -- similar to those filed by all members of Congress, stating their income sources and investment holdings in broad financial categories.
"We will comply with all the personal financial disclosure procedures required by the Federal Election Commission," said Kevin Madden, a Romney spokesman.
Madden said no final decision had been made as to whether Romney would release his tax returns, but he did not release them during his 1994 run for the U.S. Senate or his 2002 run for governor.
The financial disclosure forms are designed to provide "protection against potential conflicts of interest," said Fred Wertheimer, president of Democracy 21, a nonpartisan group that promotes openness in government.
They are not, however, audited by the government, leaving the possibility of candidates filing incomplete or inaccurate documents.
In any event, the financial disclosure documents provide only a fraction of the information available on tax returns. They do not track charitable donations, gift-giving or stock transactions.
Candidates do not have to reveal the value of their mortgages, or deductions such as medical expenses, which can reveal chronic health conditions or ongoing medical treatments.
Only tax forms would reveal whether a wealthy candidate -- many of the 2008 candidates are multimillionaires -- have used loopholes to duck taxes.
And while candidates do have to describe their sources of income, they do so only in broad categories.
For instance, when Edwards revealed how much he was paid for his year-long stint consulting for the Fortress Investment Group, he can give a rough estimate -- between $50,001 and $100,000, or between $100,001 and $1 million, for example.
Joseph J. Thorndike, a historian and contributing editor for Tax Analysts, a nonpartisan group, acknowledged that privacy concerns may push candidates to keep their tax returns private.
But releasing one's forms, he said, demonstrates that a candidate "shows trust and respect for the office," he said.
In short, Thorndike said, the tax return tells "a lot about your life."
Jennifer Rubin is an independent writer and attorney living in Virginia.