If Ambac and MBIA lose their top ratings, billions of dollars of muni bonds will be downgraded, and the guarantees that have been sold on mortgage-related securities such as collateralized debt obligations, or CDOs, will lose value.
"The destruction of the bond insurers would likely bring write-downs at major banks and financial institutions that would put current write-downs to shame," Tamara Kravec, an analyst at Banc of America Securities, wrote in a note Friday.
Kravec cut her rating on Ambac and MBIA on Friday because she thinks that ratings downgrades are "highly probable" now.
And Ambac is downgraded.