“You don’t mess with Mary Jo,” President Obama said Thursday when he nominated Mary Jo White to head the Securities and Exchange Commission. “Mary Jo doesn’t intimidate easily, and that’s important because she’s got a big job ahead of her.”

Her record seems to bear this out. Among those she's locked up, both as U.S. attorney for the Southern District of New York (which covers Manhattan) and as an assistant U.S. attorney in the Eastern District (which covers Brooklyn), are mob boss John Gotti, six al-Qaeda members involved in the 1998 Kenyan embassy bombings and Omar Abdel-Rahman, the "Blind Sheikh," whose followers bombed the World Trade Center in 1993.

The prospect of having a tough-as-nails prosecutor leading the main agency charged with policing Wall Street has excited many advocates of tougher regulations on the financial industry. "The U.S. attorney's office for the Southern District of New York is considered the crown jewel of U.S. attorney’s offices for a reason, and under Mary Jo White, was no exception," Dennis Kelleher, who heads the watchdog group Better Markets, told me. "Not only did they have an exceptionally good record of trying cases but she attracted and kept a terrific staff."

Still, some episodes in her career do raise questions about her willingness to vigorously prosecute the industry.

The cases she prosecuted against banks as U.S. attorney failed to touch large, politically influential firms, argued Gary Weiss, an investigative journalist who has written about brokerage scams that White prosecuted. Perhaps White's most famous crackdown came in the late 1990s, when she prosecuted a number of "pump and dump operations." In those cases, brokerages, often controlled at least in part by the Mafia, used cold-calls to potential investors to artificially inflate the price of extremely cheap penny stocks, which they could thus sell for a profit. As the name suggests, first they "pump" the price, and then they "dump" their holdings.

These firms, however, needed larger firms to clear their trades. According to Weiss, whose book "Born to Steal" documents the rise and fall of Mafia-run, pump-and-dump brokerages, the firms that would clear such trades "tended to be the smaller, crummier, clearing and settlement firms…You never heard of Goldman Sachs clearing for those penny stock firms."

But the one big company that did clear such trades was Bear Stearns. "They were aware of these scams, but they were amoral," Weiss says. "They did nothing to blow the whistle on these firms. They just didn’t care. And they in fact facilitated the firms in their scams by letting them issue things on Bear Stearns letterhead. It’s almost as though Bear Stearns was approving of these banks’ activities." The company was slapped with $25 million in fines from the SEC for its work with the pump-and-dump firm A.R. Baron, but neither White nor then-Manhattan District Attorney Robert Morgenthau pursued criminal charges.

That bothers Weiss, who thinks White was looking out for her post-prosecutor career rather than for the public's best interests. "Why weren’t they prosecuted? I’ve never gotten a satisfactory answer to that…I know there was an active Morgenthau investigation, but when it comes to wrongdoing by the major banks, they would much rather be representing them than prosecuting them," he alleges. "As a career builder, would you rather prosecute these guys or defend them? Obviously, they would much rather be on their side."

Since leaving the U.S. attorney's office in 2002, White has been a partner at Debevoise and Plimpton, during which time she represented both former Bank of America chief executive Ken Lewis (who faced a lawsuit from then-New York Attorney General Andrew Cuomo for his conduct during the financial crisis) and Morgan Stanley executive John Mack.

A former SEC investigator named Gary Aguierre has alleged that White, in her capacity as a lawyer for Mack, was able to block an investigation into possible insider trading when Mack was a candidate for chief executive of Morgan Stanley. Aguierre said in an interview with author Erin Arvelund that when he questioned the decision to suspend the insider trading investigation, "I was told it was for political reasons."

But others argue that White's experience with the financial sector will be an asset. Kelleher declined to comment on the specific case of Bear Stearns, but he did say that in some cases, Wall Street expertise can help regulators craft rules that help consumers.

"I was a Wall Street defense lawyer for 20 years, and that not only didn’t hurt me when I was leadership staff in the Senate, but I was much better at those jobs because of the work I had done before," Kelleher explained.