WASHINGTON (Reuters) - A senior U.S. lawmaker urged securities regulators to slow down a pending accounting change that could force banks to bring trillions of dollars back on their balance sheets, according to a letter obtained by Reuters on Friday.

Republican Rep. Spencer Bachus of Alabama said in the letter, dated July 22, that the accounting change may have serious unintended consequences.

"Changes to securitization accounting could have a dramatic impact on the economy, the capital markets and consumers seeking credit," he said in the letter to the chairmen of the Securities and Exchange Commission and the accounting rule maker, the Financial Accounting Standards Board.

Bachus said January 1, 2010 would be a more realistic deadline than this year for finalizing accounting changes as it would allow "stakeholders" to have the time to debate alternatives and consequences.

FASB is working on a proposal to eliminate certain off-balance sheet entities called qualified special purpose entities (QSPEs) used to pool debt such as mortgages and student loans.

The board plans to issue the proposal in August despite concerns that banks and other financial firms like Freddie Mac (FRE.N: Quote, Profile, Research, Stock Buzz) and Fannie Mae (FNM.N: Quote, Profile, Research, Stock Buzz) would have to raise billions in capital at a difficult time.

The SEC had no comment and does not comment on letters. Calls to FASB were not immediately returned.

(Reporting by Rachelle Younglai; Editing by Brian Moss and Tim Dobbyn)