Carolyn Kaster/Associated Press

Mary Jo White, who appears set to become a top Wall Street regulator, is seeking to allay skepticism about her spins through the revolving door between government service and private practice.

Yet even as she confronts concerns about her work for JPMorgan Chase in financial crisis cases and Morgan Stanley’s board in vetting a chief executive, she is turning to a colleague who has trod a similar path.

Andrew J. Ceresney, who served as Ms. White’s lieutenant as both a defense lawyer and as a federal prosecutor in Manhattan, is a leading candidate to ultimately become her enforcement chief at the Securities and Exchange Commission, according to people briefed on the matter. A Washington outsider and relative unknown beyond legal circles, Mr. Ceresney would help set the tone for policing financial fraud, effectively making him a top cop on Wall Street.

He could join as soon as spring, potentially serving as co-chief with the agency’s acting head of enforcement, the people briefed on the matter said.

But the job is not yet his. Ms. White, who is expected to sail through a Senate confirmation hearing on Tuesday, must first confront questions about her ties to Wall Street. And Mr. Ceresney (pronounced Sir-rez-ney) could heighten such scrutiny.

At the S.E.C., Mr. Ceresney, 41, would have to police some of the same firms he spent a decade defending.

After working at the United States attorney’s office in Manhattan, he built a lucrative legal practice at Debevoise & Plimpton. At the firm, he represented the likes of Kenneth D. Lewis, former chief executive of Bank of America who faced regulatory investigations over the bank’s hasty takeover of Merrill Lynch during the depths of the financial crisis.

Mr. Ceresney, who defended JPMorgan from federal inquiries into wrongful foreclosure practices, also worked side by side with clients under scrutiny for selling troubled mortgage securities at the height of the housing boom.

His career path mirrors that of Ms. White, who alternated between Debevoise and the federal government for three decades.

During stints as a federal prosecutor in Brooklyn and later as the first woman to be United States attorney in Manhattan, she helped oversee the prosecution of the crime figure John Gotti and directed the case against those responsible for the 1993 World Trade Center bombing. She left in 2002 with the reputation as a tenacious prosecutor with an independent streak.

Since then, at Debevoise, she has represented nearly every big bank on Wall Street. Her clients included JPMorgan Chase, UBS and Michael Geoghegan, the former head of HSBC.

To avert potential conflicts stemming from her work on behalf of Wall Street giants, Ms. White had already agreed to recuse herself for one year from most matters involving former clients. Ms. White also vowed “as far as can be foreseen” never to return to Debevoise and plans to soon cut financial ties with the firm.

Some Senate Banking Committee members are not satisfied. Senator Sherrod Brown, Democrat of Ohio, has questioned whether Ms. White’s pledge to avoid matters involving former clients would undercut her ability to run the agency. Mr. Ceresney would probably follow Ms. White’s lead and recuse himself from certain cases, an important ethical move but also a potential hindrance to his authority.

The questions surrounding the revolving door illustrate how, even as Ms. White and Mr. Ceresney prepare to usher in a new era at the S.E.C., the agency is dogged by old concerns. While the agency has mounted cases against Goldman Sachs and other Wall Street giants, consumer advocates continue to complain that the enforcement unit remains too timid.

But there is another school of thought on the revolving door — that it actually bolsters prosecutorial instincts. Anyone fit to shine a light on the darkest corners of Wall Street, S.E.C. officials say, must navigate its ins and outs. The arguments echo claims that Franklin Delano Roosevelt picked the famed financier Joseph P. Kennedy as the first S.E.C. chairman because Kennedy “knew where the bodies were buried.”

Mr. Ceresney could also breathe new life into the enforcement unit as it enters a critical phase.

Some enforcement officials, according to people briefed on the matter, are seeking a leadership overhaul after the recent departure of Robert Khuzami, who revamped the unit after it missed years of warning signs of the huge Ponzi scheme operated by Bernard L. Madoff.

“Coming out of Madoff, you needed a guy like Khuzami who was going to reform the place,” said Thomas A. Sporkin, a senior S.E.C. enforcement official until last year when he departed for BuckleySandler. “This next person has to boost morale and broaden the agenda.”

Mr. Ceresney, however, is unlikely to present a radical shift from his predecessor, given their similar résumés. Mr. Khuzami cut his teeth prosecuting terrorists for Ms. White when she was the United States attorney in Manhattan.

Mr. Ceresney’s potential arrival also coincides with the tenure of another protégé of Ms. White, George Canellos, who became the S.E.C.’s interim enforcement chief last month. The pair, believed to be friends, are expected to overlap for a while, people briefed on the matter said.

Mr. Ceresney, a Yale Law School graduate who clerked for Judge Michael Mukasey, a former United States attorney general who is now a partner at Debevoise, would also inherit a deep bench of investigators. David P. Bergers, director of the S.E.C.’s Boston office who slid into the deputy role under Mr. Canellos, could continue as the enforcement unit’s No. 2. Matthew T. Martens, the enforcement division’s chief litigation counsel, and Daniel Hawke, who leads the market abuse unit, are also among the officials who have received consideration for more senior roles.

An S.E.C. spokesman declined to comment, as did a spokeswoman for Debevoise.

The news about Mr. Ceresney comes as the American Bar Association’s annual conference on white-collar crime gets under way. More than 1,000 criminal defense lawyers and government prosecutors from across the country have convened at the Cosmopolitan Hotel in Las Vegas for three days of symposiums and panels. Among the sessions is “The New Landscape of Insider Trading Cases,” featuring Mr. Khuzami, the outgoing enforcement chief.

Colleagues say Mr. Ceresney, who lives in Brooklyn with his wife and two children, is a brilliant lawyer and note that, unlike Mr. Khuzami, he is not an imposing figure. One former colleague described him on Wednesday like this: “short in stature but long on brains.”