[ By Howard Haykin ] Citigroup, the 3rd-largest U.S. lender by assets, faces the real possibility of a two-level downgrade of its credit by all three major credit rating firms.  Such an occurrence would mean a huge financial burden to the bank, which would be compelled to come up with $4.7 billion in cash - just to cover additional collateral for derivative triggers and exchange margin requirements.  The sum would be $1.1 billion if only Moody’s Investors Service takes action. Those estimates, reported by Citigroup in its quarterly filings with regulators, are based on holdings as of 3/31/12 and include the parent company and Citibank N.A.  Citi said it has about $421 billion in liquidity to meet those obligations, and could also consider "selling or financing highly liquid government securities, tailoring levels of secured lending, adjusting the size of select trading books, reducing loan originations and renewals, raising additional deposits" or borrowing from central banks. Moody’s, which is reviewing banks and securities firms with global capital markets operations, has said it’s considering downgrades of the world’s biggest lenders, including a two-level cut for Citigroup.  While analysts have said the change was expected, it nevertheless would sting Citigroup's bottom line, and reputation.   [NYPost, 5/4/12]