Under New Chief, a Feistier S.E.C. Emerges First the Securities and Exchange Commission rejected a settlement with a high-flying hedge fund manager, Philip A. Falcone. Then it charged another billionaire trader, Steven A. Cohen. By late Friday afternoon, it had accused one of the nation's largest cities, Miami, of securities fraud. It was a busy 24 hours for the S.E.C., the federal regulator once blamed for missing the warning signs of the financial crisis and the vast Ponzi scheme orchestrated by Bernard L. Madoff. The flurry of moves appeared to signal that the agency was striking a harder line with Wall Street under its new chairwoman, Mary Jo White. While it is still early in her tenure, and the agency faces lingering criticism for its close ties to Wall Street, Ms. White has taken several steps to crack down on financial fraud. "They're now demonstrating an aggressiveness that is highly unusual," said Thomas A. Sporkin, who spent nearly 20 years in the S.E.C's enforcement unit until last year, when he moved to the law firm Buckley Sandler. It's rare to see a day like today. When Ms. White was nominated in January, some politicians and consumer groups expressed concerns about her connections to Wall Street. A former federal prosecutor turned defense lawyer, Ms. White has repeatedly spun through the revolving door connecting government and private practice. During her confirmation, several questioned whether Ms. White, who spent the last decade representing big banks like JPMorgan Chase and UBS, could have conflicts of interest. Her recent actions have started to assuage some concerns. Already, Ms. White has moved to address a central criticism of the agency: that it allows defendants to neither "admit nor deny" wrongdoing when reaching settlements. The leaders of the S.E.C. enforcement unit detailed the policy shift in a memo last month, saying there might be cases that "justify requiring the defendant's admission of allegations in our complaint or other acknowledgment of the alleged misconduct as part of any settlement." "It's welcome news for the American people desperate for a tougher S.E.C.," said Dennis M. Kelleher, who runs Better Markets, an advocacy group critical of Wall Street. He said, however, that the agency still had a high bar to prove it could be a tough enforcer. "Two hedge fund cases are good, but not good enough," he said. A preliminary settlement with Mr. Falcone had been collapsing for weeks, people close to the S.E.C. said, as Ms. White and the agency's other commissioners questioned whether it was too lax. On Thursday, the agency's commissioners rejected the settlement, a rare move that happens only once or twice a year. Moments later, the S.E.C. notified Mr. Falcone and his hedge fund, Harbinger Capital Partners, that the agency had rejected "the previously disclosed agreement in principle," according to a public filing his company made on Friday. The charges stemmed from accusations that Mr. Falcone had manipulated the market, used hedge fund assets to pay his own taxes and secretly favored select customers at the expense of others. The S.E.C."s rejection of the settlement - a move that will prompt the agency to either negotiate a tougher penalty or take Mr. Falcone to trial - suggested that its preliminary deal did not match the gravity of the crime. The deal, announced in May by Mr. Falcone, came with an $18 million penalty from the S.E.C., a rounding error to a hedge fund billionaire. While the deal also included at least a two-year ban from raising new capital, that punishment came with a number of caveats. And in a moral victory for Mr. Falcone, the deal also omitted a common provision barring defendants from committing future violations with fraudulent intent, raising concerns that the S.E.C."s results fell short of its ambitions. For a time, the S.E.C. was questioning whether to sanction Mr. Cohen. The agency spent nearly a decade investigating his hedge fund, SAC Capital Advisors, and even brought charges against several employees. But Mr. Cohen was not accused of wrongdoing. That changed on Friday, when the S.E.C. accused him of "failing to supervise" employees. The action, filed as an administrative proceeding at the agency rather than as a lawsuit in federal court, delivers a serious blow to Mr. Cohen. The agency is seeking to bar him from overseeing outside investor funds, a death knell to a hedge fund manager. It is unusual for the S.E.C. to pursue a case against someone of Mr. Cohen's stature without formally accusing him of insider trading or fraud. The charge of failing to properly supervise is similar to what other regulators have done in a lawsuit against Jon S. Corzine, who led MF Global during the brokerage firm's collapse two years ago. In its charging document on Friday, the S.E.C. says Mr. Cohen failed to halt two of his portfolio managers from trading on confidential information. The two SAC employees, who both face criminal charges, were swept up in a broad federal investigation into insider trading. One of the portfolio managers, Mathew Martoma, is accused of improperly acting on data about a clinical drug trial in 2008. The other, Michael S. Steinberg, is accused of trading on confidential information about Dell's financial performance that same year. Both men have denied the charges and face separate trials that begin in November. An SAC spokesman said that the S.E.C."s action had no merit. "Steve Cohen acted appropriately at all times and will fight this charge vigorously," the spokesman said. The case against Miami came just hours after the action against Mr. Cohen was announced. The S.E.C. accused the city of giving misleading information about its finances to investors in 2009 in an effort to make its municipal bonds more attractive. The agency also said the city broke a cease-and-desist order it signed in 2003 after facing similar charges. George Canellos, co-director of the S.E.C."s enforcement unit, said in a statement that the city's conduct was "all the more appalling and unacceptable" because of the earlier problems. A lawyer for Miami, Ivan Harris, said the city would fight the charges in court.