WASHINGTON - Sen. Hillary Clinton excluded nearly $24 million of her husband's earnings from Senate financial statements from 2004 through 2006, capitalizing on rules that permit senators to limit disclosures of some of their spouses' income.

Her decision, while fully consistent with Senate rules and norms, delayed the release of financial information about former President Clinton's soaring income until the couple released their tax returns in early April, under pressure from Democratic presidential rival Sen. Barack Obama. By then, about 40 states had completed their Democratic primaries and caucuses, meaning that those voters didn't get a clear look at Bill Clinton's finances.

Like Clinton, Obama listed his wife Michelle's salary and directors' fees only as "over $1,000," which complies with Senate rules. Obama and his wife, a Chicago lawyer, aren't as wealthy as the Clintons, however, and their finances are less murky.

GOP candidate John McCain's wife, Cindy, is the heiress to a beer distributorship and has owned stock in oil and pharmaceutical companies, but the specifics are elusive. McCain has declined to release his wife's tax returns, saying they keep their finances separately.

Watchdogs say these scenarios not only raise issues about the candidates' openness but that they also point to shortcomings in government ethics requirements.

Bill Buzenberg, the executive director of the Washington-based Center for Public Integrity, said that the disclosures by the Clintons and McCain were "inadequate."

"There's no other way to sugarcoat it," he said. "It's not transparent. It needs to be because it does potentially involve all kinds of entangling things we don't know."

The Clintons' tax returns show that Bill Clinton earned nearly $51 million from 2004 through 2006. His wife informed the Senate of about $27 million of it, consisting almost entirely of fees from his globe-trotting speaking tours, from which he has fetched as much as $400,000 for a single appearance.

Reporting rules for senators and presidential candidates allowed Hillary Clinton to describe the amounts of her husband's other income sources as "over $1,000." These included his more than $10 million in advances and royalties from two book deals, as much as $11.5 million from offshore partnerships that invested in a Chinese media company and more than $2 million from a Nebraska firm whose chairman reportedly spent $900,000 flying the Clintons aboard corporate jets for personal, business and campaign trips.

The sketchy disclosures on Clinton's statements might help explain why many Americans were surprised to learn, upon release of the couple's tax returns and a summary of their 2007 income, that they earned $109 million over the last eight years after leaving the White House buried in debt in 2001.

Jay Carson, a spokesman for Clinton's presidential campaign, said that like Obama, Clinton "reported all of her spouse's income in accordance with Senate ethics requirements, which are clearly spelled out." The rules require less information about spouses' income.

On at least three occasions in February, however, Clinton campaign officials reassured the public that her Senate disclosure forms offered a sufficient look into her finances.

"She has released, as part of the financial disclosure process as a senator, sources of her revenue every year she has been in office," Phil Singer, a campaign spokesman, told the Christian Science Monitor.

Obama was the first candidate to release a tax return, making public his 2006 filing early last year and stating that he wanted to set a new standard for openness.

In March he released full returns for the years 2000 to 2006 and beckoned Clinton to do the same. Obama's 2005 return revealed that his wife was paid $33,000 in directors' fees for Bay Valley Foods and $12,000 for serving on the board of Treehouse Foods.

In his Senate financial statements from 2004-2006, Obama reported that he and his wife earned about $1.4 million, mostly from his two books. In contrast, the couple's tax returns for those years, which include their salaries, listed gross income of about $2.9 million.

McCain didn't release his 2006 and 2007 tax returns until last month, after he had essentially locked up the Republican presidential nomination.

The returns don't mention his wife, but in a disclosure statement required of presidential candidates, McCain reported last year that his wife and their children had 2006 income of $2.3 million to $7.1 million from land in Arizona and California, family trusts and securities.

McCain's returns show that he earned $358,414 in 2006 and $386,527 in 2007. But his Senate statement put his 2006 income at $150,000 to $1.1 million, underscoring the vagueness of government financial statements, which list income and asset values in wide ranges.

Hillary Clinton's financial statements for the years 2001 through 2003 were more consistent with the couple's tax returns because her husband's income in his first years outside the White House came mostly from speaking fees that senators must disclose.

The financial gap between her Senate reporting and her husband's 2004-2006 earnings coincided with a period in which Bill Clinton began to enter private business deals that caused the couple's wealth to mushroom.

Watchdogs said this suggests the need to strengthen Senate reporting requirements aimed at guarding against conflicts of interest.

David Epstein, a Columbia University political science professor who specializes in congressional ethics, said he thinks that politicians shouldn't be compelled to release their tax returns every year, but "that's the way things work" because government disclosure rules are so weak that there's a clamor for further disclosure.

"My understanding was that Hillary was following the norms of the day in what she had to release," he said. "But does that say something about how easy it is for politicians to do favors and get away with it? Sure. ... There may be something wrong with the reporting requirements. The Congress is notoriously lax about policing itself."

The Center for Public Integrity's Buzenberg called the 1984 flap over the business activities of Democratic vice presidential candidate Geraldine Ferraro's husband, New York real estate mogul John Zaccaro, "small potatoes compared to the millions and millions (of dollars) we're talking about with the former president."

He noted that Bill Clinton's presidential library in Little Rock, Arkansas, has refused to identify many of its donors.

The Clintons have taken other steps to comply with ethics standards, such as putting their investments into blind trusts and then selling their stocks -- including shares in oil, chemical and pharmaceutical firms -- last year as Hillary Clinton formally launched her presidential candidacy.