“You don’t want to mess with Mary Jo.”

That’s what President Obama said about his pick to run the Securities and Exchange Commission, Mary Jo White. The nomination of Ms. White, a former prosecutor who took on the terrorists behind the bombing of the World Trade Center in 1993 and the Mafia boss John Gotti, was meant to signal that the S.E.C. would be getting tough on Wall Street. CBS called her “Wall Street’s new sheriff.” The Wall Street Journal said she would be “putting a tougher face on an agency still tainted by embarrassing enforcement missteps in the run-up to the financial crisis.” The New York Times said her appointment represented a “renewed resolve to hold Wall Street accountable.”

Hold on.

While Ms. White is a decorated prosecutor, she has spent the last decade vigorously defending — and billing by the hour — Wall Street’s biggest banks, as a rainmaking partner at the white-shoe law firm Debevoise & Plimpton. The average partner at the firm was paid $2.1 million a year, according to American Lawyer; but she was no average partner, very likely being paid at least double that. Her husband, John W. White, is a corporate partner at Cravath, Swaine & Moore. He counts JPMorgan Chase, Credit Suisse and UBS as clients. The average partner at Cravath makes $3.1 million. He, too, was a former official at the S.E.C. — he left Cravath to run the corporate division of the S.E.C. starting in 2006 just in time for the run-up to the financial crisis. He left in November 2008, a month after the bank bailouts, to return to Cravath.

It seems Mr. and Ms. White have made a fine art of the revolving door between government and private practice.

So how conflicted is Ms. White? Let’s count the ways.

They are well documented: she was JPMorgan Chase’s go-to lawyer for many of the cases brought against it relating to the financial crisis. She was arm-in-arm with Kenneth D. Lewis, Bank of America’s former chief executive, keeping him out of trouble when the New York attorney general accused Mr. Lewis of defrauding investors by not disclosing the losses at Merrill Lynch before completing Bank of America’s acquisition of the firm. (And empirically, Mr. Lewis did keep crucial information about the deal from investors.)

This is what she had to say about Mr. Lewis, in a court filing submitted on his behalf: “Some have looked to assign blame for every aspect of the financial crisis, even where there is no evidence of misconduct. This case is a product of that dynamic and does not withstand either legal or factual scrutiny.” It was a refrain she often made about her clients related to the financial crisis.

And then there was Senator Bill Frist, the Republican from Tennessee, whom she successfully represented when the S.E.C. and the Justice Department started an investigation into whether he was involved in insider trading in shares of HCA, the hospital chain. She persuaded them to shut down the investigation.

She also worked with Siemens, the German industrial giant, when it pleaded guilty to charges of bribery, paying a record $1.6 billion penalty.

And then, of course, there was John Mack. She worked for the board of Morgan Stanley during a now well-publicized 2005 investigation into insider trading that ended soon after she made a phone call to the S.E.C. Using her connections at the top of the agency, she dialed up Linda Thomsen, then the commission’s head of enforcement, to find out whether Mr. Mack, who was being considered for Morgan Stanley’s chief executive position, was being implicated. He ultimately wasn’t. As the Huffington Post pointed out in a recent article about Ms. White, Robert Hanson, an S.E.C. supervisor, later testified, “It is a little out of the ordinary for Mary Jo White to contact Linda Thomsen directly, but that White is very prestigious and it isn’t uncommon for someone prominent to have someone intervene on their behalf.”

All of Ms. White’s previous engagements create not only an “optics” problem, but a practical, on-the-job problem. She will most likely need to recuse herself from just about anything related to her previous work.

“I will not for a period of two years from the date of my appointment participate in any particular matter involving specific parties that is directly and substantially related to my former employer or former clients, including regulations and contracts,” is the language in an ethics pledge that she will have to agree to follow.

Some appointees, including Mary L. Schapiro, the former chairwoman of the S.E.C., recused themselves from any involvement in work that was related to a previous employer even after the two-year moratorium. Gary Gensler, the chairman of the Commodity Futures Trading Commission, recused himself from the investigation into MF Global because of his previous employment at Goldman Sachs, where Jon Corzine was the firm’s head, even though it had been years since the two had worked together.

And then there is the issue of Mr. White’s husband, who will have a continuing role at Cravath, one of the most pre-eminent firms in the country, whose clients include some of the nation’s largest corporations.

“This president has adopted the toughest ethics rules of any administration in history,” said Amy Brundage, a White House spokeswoman, “and this nominee is no exception. As S.E.C. chair, Mary Jo White will be in complete compliance with all ethics rules.”

None of these conflicts gets at another potential problem for Ms. White. The job of chairwoman of S.E.C. isn’t simply about enforcement; she has a deputy for that. The biggest challenge anyone who takes the job will have to confront over the next several years will be executing and enforcing provisions of Dodd-Frank and working to regulate electronic trading — something that even the most sophisticated financial professionals, let alone a lawyer, often have a tough time understanding. She has zero experience in this area.

Of course, there can always be a value to inviting a onetime rival onto the team.

“I believe she is one of those people who will understand that her public role will be very, very different than her role as a defense lawyer,” Dennis M. Kelleher of Better Markets, a watchdog group, told me. “I don’t think she’s going to be like so many others who don’t get that they have a very different role when they hold high public office.

“No question, she’s said some things that are controversial and questionable,” Mr. Kelleher said. “Moreover, I hope and expect that she will be asked publicly about them in the confirmation process and that she will have convincing answers.”

Of course, if she is confirmed, we must all hope that she can put her previous client relationships behind her and work for her new client — us.