Mark Pittman, Alan Katz and David Mildenberg / Bloomberg.com

Bank holding companies including Citigroup Inc., Bank of America Corp. and Wells Fargo & Co. have the thinnest safety cushion against losses in seven years.

The margin may erode further in coming weeks. Credit ratings on $704 billion of bonds have been cut this year following the collapse of the U.S. housing market. Sheila Bair, chairman of the Federal Deposit Insurance Corp., said last week that downgrades may compromise bank capital ratios enough that some of the largest institutions will no longer be considered well capitalized.

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