A once-obscure accounting rule that infuriated banks ... was changed Thursday to give banks more discretion in reporting the value of mortgage securities. The change seems likely to allow banks to report higher profits by assuming that the securities are worth more than anyone is now willing to pay for them. But critics objected that the change could further damage the credibility of financial institutions by enabling them to avoid recognizing losses from bad loans they have made. Critics also said that since the rules were changed under heavy political pressure, the move compromised the independence of the organization that did it, the Financial Accounting Standards Board. During the financial crisis, the market prices of many securities, particularly those backed by subprime home mortgages, have plunged to fractions of their original prices. That has forced banks to report hundreds of billions of dollars in losses over the last year, because some of those securities must be reported at market value each three months, with the bank showing a profit or loss based on the change. At first FASB ... resisted making changes, but that changed within a few days of a Congressional hearing at which legislators from both parties demanded the board act. There is a perception that we are yielding to political pressure," one board member, Lawrence W. Smith, said as he voted for the changes. A group headed by two former chairmen of the Securities and Exchange Commission, one who served under President Bill Clinton and one who was appointed by President George W. Bush, said that it feared that politicization of accounting standards would destroy the credibility of the board.