HARRISBURG — Defense lawyers for State Rep. Vanessa Lowery Brown are asking the judge in her political corruption trial to dismiss all charges before the case goes to the jury.

The request came Monday as Brown's lawyers began presenting her defense in the case, which grew out of a sweeping sting investigation run by the state Attorney General's Office between 2010 and 2012. The Philadelphia Democrat is accused of taking five cash payments in 2011 from the sting's undercover operative, Tyron B. Ali, in exchange for official favors.

Brown's lawyers — Patrick A. Casey, John B. Dempsey, and Erik R. Anderson — argued that prosecutors failed to show specific instances of Brown accepting money in exchange for votes. Prosecutors Michael Sprow and Ryan Shovlin contend that Brown accepted five payments totaling $4,000. They have pointed to her testimony before a grand jury that she knew she was breaking the law when she accepted the money from Ali, who at the time was posing as a lobbyist and discussing votes on various bills.

Brown's request for an acquittal, although routine for defense lawyers in mid-trial, will be closely watched. In an unrelated but high-profile political corruption case last year, a federal judge, in an extraordinary decision, dismissed all charges after prosecutors had presented their case. Still, lawyers have said a mid-trial acquittal would be rare.

Earlier Monday, four defense witnesses took the stand to attack the credibility of Ali, the trial's star witness. Brown's lawyers have sought to paint Ali as a consummate con man who spent years swindling taxpayers and unsuspecting business partners.

Two of the witnesses were Philadelphia-area investors in a nonprofit that Ali ran to help people restructure their mortgages to keep their homes. Both testified that they loaned Ali money but were never repaid in full.

One, Masoud Soroush, is a chemical-engineering professor at Drexel University who said Ali represented his business as one that aimed to help people who suffered from bad credit. Soroush told jurors that he lent Ali $311,000 in 2009, and that Ali did not make good on his end of the deal. Soroush later sued Ali, but said he only recouped about $60,000.

On cross-examination by Sprow, Soroush acknowledged that he did not know Brown or have any personal knowledge of the allegations against her.

"Do you have any personal knowledge of her taking cash bribes in her position as a state representative," Sprow asked.

"No," Soroush responded.

Also testifying were current or former state auditors whose investigation into one of Ali's business ventures led to criminal charges. Ali was charged in 2009 by the Attorney General's Office in a massive fraud case. He was accused of using a North Philadelphia daycare center he operated to defraud taxpayers of more than $400,000 intended to provide meals for low-income children and seniors.

Both auditors said they looked at canceled checks and bank statements from the day-care business, among other records, and quickly determined that documents were forged.

"The math wasn't checking out," said Martin Scripko, a retired state auditor. The matter, Scripko said, was referred to the Attorney General's Office for further investigation.

Both auditors, on cross-examination, said they were not forensic accountants or certified fraud examiners. The onetime state prosecutor who oversaw the sting investigation last week testified that the original criminal case against Ali was overblown, and that a further review from his office was able to pinpoint much of the money Ali was alleged to have stolen.

Ali began cooperating with state prosecutors after the charges were filed in the fraud case, agreeing to go undercover as a lobbyist and surreptitiously record elected officials. In all, six Philadelphia Democrats — including five former or current lawmakers — were charged. Brown is the only one to fight the charges. The others pleaded guilty or no contest. All charges against Ali in the fraud case were eventually dropped.

The sting case has divided the state's legal and political community for years, with some critics contending it was poorly managed and targeted at Democrats from Philadelphia.

On Monday, Brown's lawyers read a sworn statement to the jury from an FBI agent who said Ali told him he was discouraged from targeting Republicans. The agent, Robert J. Haag, said Ali told him that he after he contacted GOP lobbyists and public officials, he was "reprimanded."

State prosecutors have repeatedly rejected the idea that Ali was told to steer clear of Republicans. During the investigation, they said, they told Ali to avoid certain officials for fear his cover would be blown.