What will the long-term impact of the downgrade be?

A number of entities that are key players in the U.S. financial system — including mortgage finance companies Fannie Mae and Freddie Mac, and securities clearinghouses like the Options Clearing Corp Depository Trust Co — are likely to be downgraded by Standard & Poor’s on Monday.

For Fannie Mae and Freddie Mac, losing their triple-A rating could lift borrowing costs, potentially making mortgages more expensive for consumers and adding to stress in the already unstable U.S. housing market…

Another source of potential stress is derivatives markets, where investors and banks often collateralize their positions using U.S. Treasuries.

If banks start demanding more Treasuries to collateralize the same exposure, investors could be forced to sell assets to come up with extra collateral, causing broader market declines. As long as Treasury yields are at all time lows, that risk seems relatively low, said a hedge fund trader who spoke on condition of anonymity.