Bankers long have complained the tax break is an unfair advantage for large credit unions. Now they see an opportunity to get rid of it as lawmakers begin work on a major overhaul of the tax code that is aimed at eliminating many corporate exemptions and lowering the overall tax rate.The exemption cost $1.6 billion this year in taxes avoided and would rise to $2.2 billion annually in 2018, according to Obama's proposed 2014 budget.In a 2010 report on tax reform, the President's Economic Recovery Advisory Board said eliminating the exemption would raise $19 billion over 10 years and remove the credit unions' "competitive advantage relative to other financial institutions" in the tax code.Credit unions said the effort to take away their tax exemption was simply an attempt to stifle competition and remove one of the only checks on bank fees for consumers.And it comes as some in Congress are pushing to loosen regulations on credit unions so they can expand their business further, including legislation that would lift a cap on the amount of money they can lend to businesses.The tax exemption is crucial to credit unions, which by law can't raise capital through public stock offerings the way that banks can, said Fred R. Becker Jr., president of the National Assn. of Federal Credit Unions, a trade group with about 3,800 federally chartered members."They'll have to convert to banks, which is what the banks want," he said. "Then they'd have, for lack of a better term, a monopoly."