The two federal cases that mesmerized Albany this week — a state senator accused of trying to bribe his way onto the New York City mayoral ballot and an assemblyman charged with taking payoffs to write made-to-order legislation — come as federal prosecutors in Manhattan have increased their focus on political corruption, officials said on Friday.  The two cases, brought by the office of Preet Bharara, the United States attorney in Manhattan, and in the works for several years, developed after the office’s public corruption unit had begun to grow and use more aggressive tactics, a process that began at least four years ago, according to Richard B. Zabel, the deputy United States attorney.  The unit, which had about half a dozen assistant United States attorneys in 2009, now has 10; an additional 4 prosecutors work in a satellite office in White Plains, handling mostly political corruption cases, Mr. Zabel said. One of the White Plains prosecutors was involved in the case against the state senator, Malcolm A. Smith, a Democrat from Queens. Prosecutors in other units in the office, he said, also work at times on political corruption cases.  “The growth was because we need to increase our enforcement in this area, we need more prosecutors to have more time to look at these cases,” Mr. Zabel said.  Political corruption cases are hard to investigate because they usually involve conspiracies between two people, and unless one of them cooperates with the authorities and helps gather or provide evidence against the other, the government’s options are limited. These kinds of crimes often take place amid a constellation of regular, legal political deals that may not be easily distinguishable from those that are criminal unless investigators have inside information.  And while the world of corrupt politicians includes those whose schemes sometimes seem so poorly thought through that they make investigators scratch their heads — like the plan Senator Smith is accused of to get on the Republican mayoral ballot — it is also populated by shrewd and savvy criminals wary of law enforcement and knowledgeable about how it operates.  Mr. Bharara, sworn in as United States attorney in 2009, has said that public corruption cases are, along with terrorism and securities fraud, a top priority.  His office brings such prosecutions, and to some degree oversees the investigations that lead to them. Most often, the investigations are conducted by the Federal Bureau of Investigation and the city’s Department of Investigation, and at times by some of the 16 investigators who work for the United States attorney’s office.  In recent years, Mr. Bharara’s office, like that of the F.B.I. and his colleague across the East River, Loretta E. Lynch, the Brooklyn United States attorney, have been using increasingly aggressive tactics in public corruption investigations, as they have in securities fraud cases — tactics more traditionally associated with investigations involving drugs and organized crime.  Mr. Zabel noted that the case announced this week involving the assemblyman, Eric A. Stevenson, a Bronx Democrat, who was accused of writing legislation to benefit a businessman in exchange for bribes, was handled in part by a retired police detective, John Barry, who works as an investigator in Mr. Bharara’s office and who for years focused on international narcotics traffickers.  In that case, another assemblyman, who had been indicted on perjury charges by state prosecutors in the Bronx and was cooperating with Mr. Bharara’s office, wore a hidden recorder to gather information in the investigation, serving as a mole in the Legislature.  But it was far from the first time that such aggressive tactics reached into the Legislature itself. In 2008, an F.B.I. undercover agent, working with prosecutors in Mr. Bharara’s office who were investigating Assemblyman Anthony S. Seminerio, met with Mr. Seminerio in his Assembly office and was invited to the floor of the chamber.  George Venizelos, the assistant director in charge of the F.B.I.’s New York office, said the bureau had also increased its resources focused on public corruption cases. “They’re some of the most important cases the F.B.I. works, because it’s important that the government maintains the public trust,” he said, “because without the public trust, the government has trouble working.”
More prosecutors focused on political corruption and the use of more aggressive tactics may have helped lead to recent arrests 0f lawmakers accused in bribery cases.