A Goldman Sachs Group Inc. analyst downgraded the stock of Citigroup Inc., the largest U.S. bank in assets, to "sell," after predicting that the lender's write-downs of collateralized debt obligations will total $15 billion over the next two quarters, Bloomberg reported. The analyst dropped his estimate of Citigroup's earnings per share to $3.80 from $4.65 and his price estimate to $33 for next year. Citigroup fell $1.21, or 3.6 percent, to $32.79 at 9:05 a.m. in New York Stock Exchange composite trading. The shares have declined 41 percent this year. "Given the dislocations in the credit markets, we have become more pessimistic,' New York-based analyst William F. Tanona wrote to investors today, downgrading Citigroup from "neutral." "Citigroup will likely face an increasingly challenging operating environment which is likely to pressure results in many of their businesses." Tanona said rising mortgage delinquencies may cut into earnings.