The pressure on Citigroup is mounting.

Analysts at Goldman Sachs have downgraded their rating to "sell" on Citigroup shares, saying that the bank may have to write down as much as $15 billion on its collateralized debt obligations. Shares of Citigroup tumbled nearly 6 percent. The downgrade, and the continued slide in Citigroup's stock, may complicate the search for a new chief executive. (For more on the search, see "Can Anyone Lead Citi?")

Citigroup has estimated that its write-down may be as much as $11 billion in the fourth quarter, after taking a $6.5 billion charge in the third quarter.

"With deteriorating consumer and housing metrics, Citigroup is facing mounting pressure across many businesses," the Goldman analysts wrote, according to Reuters. "Further magnifying the earnings decline is a peer-low tier-1 capital ratio, which may lead the firm to pursue either a capital infusion or the prospect of reducing its current dividend."

Reuters notes that the Goldman downgrade follows on the heels of a report by Tobias Levkovich, Citigroup's chief U.S. equity strategist, who has raised his rating on the banking sector, calling selling pressure "overdone."

The Goldman report says that the absence of a chief executive to direct strategy "may limit trading activities, and Citigroup may be unable to either capitalize on business relationships or utilize its balance sheet."

"The lack of leadership at this point in Citi's storied history could not have come at a worse time," analysts wrote. "It will likely take the new C.E.O. some time before he or she decides on the appropriate course of action to undertake."

Charles Price stepped down as Citigroup's chief executive on November 4. Robert Rubin, the company's chairman, is leading the search for his successor.

Robert Willumstad, a former top Citigroup executive who is now chairman of the insurance giant American International Group, is said to be a leading contender, as is Vikram Pandit, a former Morgan Stanley president who now oversees investment banking at Citigroup.

The Telegraph of London reported on Sunday that Fred Goodwin, the chief executive of Royal Bank of Scotland, was asked about his interest, although the newspaper said he was unlikely to take the job.

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