Although U.S. Attorney Preet Bharara's office has brought charges against 18 state legislators and secured numerous convictions, Bharara said Friday making these kinds of cases is very difficult because of the climate of secrecy in Albany.

"People don't often appreciate that [with] certain kinds of crimes, when the event happens the crime is committed," Bharara, the top federal prosecutor in Manhattan, explained during a speech at Fordham University School of Law.

But with political corruption cases, prosecutors have to unravel the secret deals and passings of money that may be at the root of a politician's behavior, he said.

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"There is nothing illegal per se about casting a vote," Bharara said. "The question is: What was in that person's mind when he cast that vote? Was that vote cast in a particular way because money was received from an interested party that was not disclosed? Was there a quid pro quo? It is hard to prove those things."

That was about as close as Bharara came to referring to his office's current prosecutions of such politicians as former Assembly Speaker Sheldon Silver, accused last month of taking payments to support legislative action.

Faced with charges by Silver's lawyers that Bharara has polluted the grand jury process by talking about Albany corruption, the prosecutor didn't mention the Silver case or any other directly at Friday's conference on political corruption.

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Prosecutors have said in court papers that Bharara's public comments don't violate ethical rules on commenting about criminal cases.

He defended his speaking out about corruption as a way to keep the issue in the public eye and get the public to think about it.

Bharara said many people think he is pessimistic about the possibility of ethical change in Albany.

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"I am actually not pessimistic. I am actually optimistic," Bharara said. "I think the level of attention, based on the cases we have brought, have caused people to think more thoughtfully, more deeply and more actively about the ways in which we can have honest government."

"If we could clean up Times Square, can we really not clean up Albany?" Bharara asked rhetorically.

Asked by a reporter if he is optimistic about the ethics reform being pushed by Gov. Andrew M. Cuomo, Bharara said he couldn't predict the outcome.

"We will see what happens," he said.