Moody's just drove a knife into the heart of Bank of America's stock, and gave an implicit thumbs up to Citigroup.

The rating firm downgraded BofA's short- and long-term credit ratings. In part, Moody's says the downgrade resulted from the waning implicit government support for big banks including Bank of America.

The credit-rating firms have been warning seemingly forever that with the U.S. government (in theory) less likely to bail out big banks in the future, the banks' credit ratings might be too high. But Moody's hadn't pulled the trigger on its warning -- until now.

Bank of America shares are off 4.1% to $6.61. The Moody's downgrade of the long-term credit rating is a two-notch slide, to Baa1.

Wells Fargo, Citigroup, BofA, Goldman Sachs and Morgan Stanleywere on Moody's list for possible downgrades considering the government's (in theory) decreasing willingness to prop up the Too Big to Fail set. Wells Fargo's stock fell off a cliff when the Moody's headlines hit, but now they're slightly positive. Take THAT, Moody's.