When a large, public company starts shopping for a new chief executive, it's rarely a smooth, easy process, but Citigroup's C.E.O. search is turning into something of a train wreck.  

William Tanona, an analyst with Goldman Sachs, cut his rating on Citigroup shares to "sell'' today, triggering a tumble in the stock price. Among his reasons for the downgrade was Citigroup's leadership vacuum at the top.

The latest blow, the steady slide in the stock, and the expectations of further write-downs to come (Goldman says as much as $15 billion), only increases the pressure on Robert Rubin, chairman of Citigroup, who is leading the search, to find the right man or woman—quickly. Finding a chief executive for a company as large and sprawling as Citigroup would be difficult enough in the best of times. And these, clearly, are anything but the best of times for Citigroup.

To make matters worse, there is no obvious No. 2 who could step up. Under Charles Prince, who stepped down on November 4, a number of top executives left or were forced out: Robert Willumstad, Marjorie Magner, Michael Carpenter, Todd Thomson, and Thomas Maheras.

And Citi found itself in an unusual head-to-head competition, as Merrill Lynch's ouster of Stanley O'Neal, just days before Prince's departure put two bulge-bracket banks roaming the Street for talent at the same time.  

Merrill won its prize hire with John Thain, the New York Stock Exchange chief executive and former top Goldman Sachs executive, whose reputation as an operations expert made him widely viewed as the best man for a turnaround at either firm.

So with Thain out of the lineup, who is Citi considering?

Many say the current front-runner for the job is Willumstad, the former chief operating officer who was passed over for the job that Prince got. Willumstad is currently chairman of American International Group and the head of private equity firm Brysam Global Partners.

As inside candidates go, Vikram Pandit is the only name being seriously spun around the rumor mill. The former Morgan Stanley president joined the firm in April and was quickly promoted to run Citi's investment banking and alternative investment units. Pandit might have potential, but his lack of relevant experience and short tenure at Citi make him a risky choice for the C.E.O. position.

BlackRock founder Laurence Fink, whose risk management background might come in handy for Citi, is said to be under consideration.

Then some are looking for an offer to be made to Richard M. Kovacevich, a Citi alum who later became chairman and C.E.O. of Wells Fargo. Kovacevich stepped down as chief executive at that bank earlier this year—but at 64, some speculate he may be more interested in retirement than starting an undertaking of this magnitude.

British newspapers have reported that Fred Goodwin, the chief executive of Royal Bank of Scotland and a cost-cutter in the Jamie Dimon mold, has been approached to gauge his interest in the job.

Whether any of the potential candidates would be willing to take the job (other than Pandit, who would probably jump at the chance) is another open question.

As Rob Cox on the site Breakingviews points out in a tongue-in-cheek "help wanted" ad, it will be a long, hard slog to get Citi going back in the right direction, and the faint of heart need not apply.

For it is not just that Citigroup is such a big company, but a bank that is in more than 100 countries and has a diverse collection of businesses from credit cards to structured finance. It would take several lifetimes to build up the ideal experience to run the place.

The product of two decades of mergers, Citigroup has suffered from a lack of integration and underinvestment.

Prince understood that streamlining and making the businesses work together was the goal, but he could not get the battleship turned around quickly enough.

Now his successor must tackle that task and at the same time deal with the continued fallout from the collapse of subprime, as well as try to improve profitability and keep and attract management talent.

Citigroup may simply be too big to be managed effectively, and Prince's successor may end up undoing Sandy Weill's legacy and splitting up the company.

Portfolio.com readers, who do you want to see in the chief executive's office at 399 Park Avenue?