The longtime business partner of Paul Manafort, U.S. President Donald Trump's former campaign chairman, will begin his testimony for the government at Manafort's trial on fraud charges later on Monday, a defense lawyer said.The federal trial jury in Alexandria, Virginia, had heard testimony on Friday from Cynthia Laporta, who described how Manafort and his associate Rick Gates doctored financial statements and backdated loans.Manafort has pleaded not guilty to 18 counts of bank and tax fraud and failing to disclose foreign bank accounts. The charges largely predate his five months on the Trump campaign but arose from U.S. Special Counsel Robert Mueller's investigation into Russian interference in the 2016 U.S. election.'Richard Gates'Manafort lawyer Kevin Downing said Gates would be the next witness called. Prosecutor Greg Andres walked into the courtroom with a binder labeled “Richard Gates” earlier on Monday, the fifth day of the trial. Gates is expected to be a star witness in the government’s case because of his long association with Manafort.Since the trial started last Tuesday, Manafort’s lawyers have kept their cross-examinations brief and at times refrained from attempting to rebut damaging testimony in detail.But Laporta’s testimony raised the stakes for Manafort, legal experts said. Testifying under immunity, she was the first witness to admit she knew accounting maneuvers Manafort and Gates requested of her were wrong and could be crimes. One accounting trick saved Manafort $500,000 in taxes, she said.14th witnessLaporta was the 14th witness called by prosecutors before U.S. District Judge T.S. Ellis. Mueller is also investigating possible coordination between Trump campaign members and Russian officials, but the charges against Manafort do not address that.Laporta detailed multiple examples in which Manafort and Gates sought to doctor financial records, first in order to lower Manafort’s taxable income and then later to inflate his income so that he could get bank loans.Some of the maneuvers were at the request of Gates, while others implicated Manafort, Laporta testified. In one instance Manafort’s signature was on a loan agreement created in 2015 and inappropriately backdated for the 2014 tax year, she said.Similar to prior witnesses, Laporta testified that Gates and Manafort were in lockstep but that Manafort was in charge. The jury has heard how Manafort made tens of millions of dollars for political work with pro-Russian politicians in Ukraine.Big test for defenseShanlon Wu, a former attorney for Gates before he pleaded guilty in February and starting cooperating with Mueller, said Monday would be a critical test of the defense’s strategy of pinning the wrongdoing on Gates.“They must make headway in separating Gates and Manafort,” Wu said. Wu said he was speaking as an observer now that he is no longer involved in the case.