U.S. Sen. Tom Harkin, D-Cumming, is one of the 50 richest members of Congress, according to figures compiled by Roll Call. But even the magazine doubts its own methodology.

Harkin is listed as the 27th most wealthy person in Congress, based on the minimum net worth reported on financial disclosure forms.

According to the magazine:

27. Sen. Tom Harkin (D-Iowa)
$10.50 million

According to Harkin’s financial disclosure forms, his minimum net worth has essentially doubled since 2006 because of his wife’s purchase of about $5 million worth of stock in 2007.

Harkin’s office wouldn’t comment on where the money for the purchases came from, but the disclosure form indicates that his wife, Ruth, bought and sold “over $1 million” worth of stock in United Technologies Corp., where she used to be a vice president. Harkin’s forms have previously stated that his wife’s compensation from UTC included a “contractual right to receive stock in the future,” so it is possible that she took stock that was owed to her and converted it to other securities.

The assets that are listed as belonging to the Senator alone or through joint ownership have a minimum value of less than $100,000. The Harkins list no liabilities.

However, the magazine admits, “Everything that you are about to read might be wrong.”

Roll Call’s annual attempt to rank the riches of Members of Congress is hampered by one fundamental flaw: It is based on the lawmakers’ financial disclosure forms, which are extraordinarily unreliable sources of information.

The disclosure rules allow Members to report assets in broad categories, so there is no way to tell the difference between a $20 million investment and a $5 million investment. The top category on the Members’ forms is “over $50 million,” so it is impossible to accurately account for anything worth more than that — like a professional sports team, for example. There is also a gaping loophole for assets owned by the Members’ spouse or dependent children; anything worth more than $1 million in value can be reported as “over $1 million.” There is no way to tell whether that is $1.2 million or $1.2 billion.

The rules also don’t require reporting things of value that are not used to produce income — most notably any primary residence or other home that is not used for rentals. That loophole removes from most Members’ portfolios hundreds of thousands of dollars and in come cases millions of dollars worth of assets. Airplanes, fancy cars, antiques or other valuable items are not reported.

The richest member of Congress, according to Roll Call, is Sen. John Kerry, D-Mass., with a total net worth of $230.98 million.