With Wilson Sonsini Goodrich & Rosati’s John Roos set to become ambassador to Japan, the scramble to find a new CEO at Silicon Valley’s most powerful law firm has begun.

Two primary candidates have emerged: corporate rainmaker Steven Bochner and the head of litigation, Douglas Clark, according to sources familiar with the situation. And as management mulls the decision, the tired yet inevitable question about who will step into Chairman Larry Sonsini’s shoes as the face and glue of the firm must be asked. Will the person chosen — and Sonsini is expected to have a big say — be seen as his anointed successor?

It had been known in the firm for some time that Roos — who helped raise millions for Obama — might get a post in the administration. But after the initial wave of appointments passed, the summons to Japan left Wilson management working quickly to find a replacement.

At the outset, Sonsini volunteered to step in as interim CEO, a source told Going Public. Sonsini was both chairman and CEO for a long time before he handed the CEO bit to Roos in 2005. Firm spokeswoman Courtney Dorman says the firm’s bylaws state that “when the CEO is not able to serve, the chairman would step in.” Nonetheless, our source says Sonsini’s offer wasn’t heartily embraced.

Now, through discussions between firm leaders like Sonsini and the partners sitting on Wilson’s management committees, Bochner and Clark have emerged as possible candidates for Roos’ slot.

Bochner is a Wilson Sonsini lifer — he started as an associate 28 years ago. He’s a corporate lawyer and a rainmaker, with clients like Autodesk, NetApp and Sequoia Capital. He’s got a calm and firm demeanor. And it is often said that Bochner is liked and respected by most at the firm — something that probably couldn’t be said about every powerful Wilson partner.

Clark is a securities litigator who heads up the firm’s litigation practice. He’s a relative newcomer, joining the firm in 1993 from Jones Day, but he has been heavily involved in firm management and has earned praise for his ability as a manager. Clark’s got a sense of humor, though some at the firm say he can be a bit of a curmudgeon.

Odds are looking good for Bochner. After all, Wilson’s a firm long-dominated by corporate lawyers. In fact, no one from the litigation side has ever been named a managing partner, CEO or chairman at Wilson Sonsini. On the down side for Bochner and the firm: The guy has a big practice and would most likely have to give some of it up to take on the duty of herding Wilson partners.

Although as a litigator he might seem like a long shot, Clark has been more of an administrator than Bochner in recent years. Also, could this be a good time for Wilson to gives its litigators a little boost, considering that they’re probably helping carry the firm through the recession?

Just how the final decision will be made is a little unclear. People familiar with the process say that the board of directors, the policy committee and Sonsini will all get to weigh in on the candidates.

Of course, the position of CEO at Wilson Sonsini carries more with it than being able to run a $500-million-a-year business. Lingering in the back of everyone’s mind is the issue of who will succeed Sonsini, who is not only the chairman, but a huge rainmaker whose name is synonymous with the firm.

At a healthy 68 years of age, Sonsini is probably not ready to retire to that little island off the coast of San Mateo just yet. And his offer to step in as interim CEO suggests he’s in no hurry to hand over the keys.

At the same time, the fact that Wilson partners didn’t go running to Sonsini and take his offer to fill in as CEO shows that there is some will among partners to plan for the future.

Does that mean the pick for CEO will be the one groomed to replace Sonsini? Opinions are divided.

One former partner told us that Roos wasn’t next in line just because he was picked as CEO.

“I don’t believe it was anticipated that Roos would succeed Larry,” the former partner said. “He was much more the prime minister than the king.”

Whatever the significance of the pick, it will have to be made sometime soon.

Wilson spokeswoman Courtney Dorman, who was on vacation, e-mailed us to say, “We have an executive chairman, experienced board and management team, and a succession process. If John is confirmed, we will announce our plans at that time.” When we got her on the phone, we asked her about Bochner and Clark being top contenders. “That’s highly speculative,” she replied.

Sonsini, reached briefly before heading into a meeting, referred questions to Dorman. Bochner and Clark didn’t return calls seeking comment.