The Treasury Department also estimated that the credit union tax exemption will cost $24.017 billion between 2019 and 2028.

In the report, the Treasury Department also reported that there are more than 50 tax expenditures larger than the credit union tax exemption.

The congressional Joint Committee on Taxation recently estimated that the credit union tax exemption will cost the federal government $9.9 billion between 2018 and 2022—still less than the administration's estimate.

The Treasury Department's estimate is based on the Trump Administration's economic assumptions, while the joint tax committee's estimate is based on economic assumptions made by Congress.

The tax expenditure figures are used by policymakers when they are writing tax legislation.

The credit union tax exemption came under fire this year, when Senate Finance Chairman Orrin Hatch (R-Utah) questioned whether it was outdated, saying there is little difference between credit unions and banks.

Hatch asked the Internal Revenue Service to require federal credit unions—at least the largest ones—to file informational tax returns.

IRS officials did not agree with Hatch's request.

Hatch is retiring at the end of this year and so far, no other member of Congress has made similar requests.

However, it is a perennial issue for banking groups that contend that credit unions have an unfair competitive advantage because of the tax exemption.