Missouri Senate candidate John Brunner has accused his primary opponent Rep. Todd Akin (R) of being dishonest about his finances.

But Brunner, a businessman worth as much as $87 million, some of which is housed in two offshore accounts, may soon see those attacks turned on him.

Between a private aircraft and bank accounts based in Bermuda and the Cayman Islands, where he also has a vacation home, Brunner’s personal finances could also be a liability heading into one of the tightest Senate races of the year.

Brunner and Akin are locked in a competitive primary battle with former state Treasurer Sarah Steelman to challenge Sen. Claire McCaskill, Democrats’ most vulnerable incumbent.

With a week before the Republican primary, a poll showed McCaskill trailing all three of her GOP opponents; Brunner led handily.

Brunner has recently made family assets an issue by attacking Akin, whose minimum net worth is a mere $210,000 according to a report filed in May, for his stake in family land near a newly upgraded highway.

Brunner’s campaign is running advertisements accusing Akin of securing millions in federal funding for the highway project to increase the value of the acre plot and has said the six-term lawmaker has tried to hide his stake in the land.

Akin’s campaign called the Brunner attack “half the truth” in a post on his website. A spokesman said the lawmaker does not own enough of the property to make legal decisions about its use.

The McCaskill campaign has not been shy about inserting itself in the Republican primary and is poised to pounce on Brunner’s fortune should he win his party’s nomination.

Among the four candidates, Brunner, who has given almost $7 million of his own money to his campaign, is by far the wealthiest, according to a personal financial disclosure form filed earlier this month.

Congressional candidates are required to provide the same detailed information about their finances as sitting lawmakers, including stock holdings, financial transactions and loans, but the forms provide only a rough guide to actual wealth because assets and liabilities are reported in broad ranges of value. Roll Call subtracts the total minimum value of all liabilities from the sum of the minimum value of all assets to arrive at a minimum net worth, which for Brunner is $19.7 million. At the top range of all assets he could be worth as much as $87 million.

Brunner derives the majority of his wealth, at least $9 million, from his former company Vi-Jon, a cosmetics and health care manufacturer. He also holds a minimum of about $5 million in gold and silver. But beyond that, a few unusual investments have caught the attention of his opponents.

He recently dissolved an account worth $1 million to $5 million with Edelweiss Holdings, a Bermuda-based investment company that focuses “on the preservation of wealth against the erosion of the purchasing power of money,” according to its website. His wife retains a similar account with the firm valued at more than $1 million.

A spokesman for Brunner said the candidate pays all U.S. taxes and said the account provided him “no tax advantage.” Still, offshore accounts in countries such as Bermuda with limited disclosure requirements and few, if any, national taxes, raise red flags among tax professionals.

“If people have $5 million and they are sprinkling it around the subtropical tax havens, one wonders why,” said Frances Hill, a professor specializing in tax and election law at the University of Miami Law School. “Why does a candidate for Congress want his or her money not in the U.S.?”

Jeffery Trinca, a tax attorney at Van Scoyoc Associates who served as chief of staff for the Senate committee charged with restructuring the IRS in the 1990s, said that many investors are drawn to these kind of accounts to avoid IRS reporting requirements.

Brunner also holds a “non interest bearing” checking account in the Cayman Islands, where his family owns a condominium.

The account was not reported on his personal financial disclosure form because it holds less than $5,000, according to his campaign.

“It isn’t necessarily nefarious, but it raises questions,” Trinca said. “Generally, a lot of travel to the Caymans and an account there as well tends to raise eyebrows at the IRS and the Treasury.”

Still, the Brunner campaign continues to hammer this message, pointing to a statement made by Akin’s spokesman last year that the Congressman stands to gain financially from the sale of the land near the highway.

“Missouri voters should be extremely troubled that Congressman Akin won’t accept responsibility for his own assets,” said Todd Abrajano, a spokesman for the Brunner campaign.

The spat has not been lost on the Democrats.

In a pre-emptive move, the Missouri Democratic Party filed a complaint last fall with the Federal Election Commission alleging that Brunner delayed his official candidacy to avoid disclosing his campaigns expenditures. Behind the scenes, the campaign is raising questions about Brunner’s assets and management of his company.

McCaskill, one of the wealthiest Members of Congress, is no stranger to financial scrutiny.

Republicans skewered her last year for failing to pay $287,000 in property taxes in 2011 on her own private aircraft.

She has since sold the “damn” plane.

Correction: July 31, 2012

An earlier version of this article misstated the year in which the Democratic Party of Missouri filed a complaint with the Federal Election Commission.