The most anticipated witness in the first trial to come from special counsel Robert Mueller's Russia probe could take the stand as early as Monday. It's a crucial development for President Donald Trump's former campaign chairman Paul Manafort, whose lawyers have placed his longtime business partner, Rick Gates, at the heart of the fraud and conspiracy trial.

Gates, 46, has already pleaded guilty to lying to investigators and conspiracy against the United States and has agreed to fully cooperate with Mueller's team, making him a valuable voice for the prosecution.

But Manafort's lawyers, too, have utilized Gates in their argument, framing him as the donkey on which to pin all willful wrongdoing. Manafort's lawyers contend that it was Gates who intentionally misled Manafort to embezzle money.

Prosecutors spent the first week of the trial leading more than 12 witnesses and dozens of exhibits through the courtroom, seeking to show that Manafort broke U.S. finance laws to maintain a lavish lifestyle. Gates was Manafort's right-hand man for years during the former's career as a lobbyist and political consultant in Ukraine for the pro-Russian Party of Regions.

Some experts think Mueller's team faces risks in bringing Gates to the witness stand. He could be viewed as an unreliable source who can't be trusted to tell the truth.

"I think it's a little bit risky" for the prosecutors, said Barbara McQuade, a former U.S. attorney in Michigan, who believes Mueller's team might be able to win without calling Gates to testify. "He has to admit he lied," and could be viewed as an unlikable figure by the jury, she said.

Defense attorneys posed the trial as a test of credibility between Manafort and Gates. In his opening statement last week, Manafort lawyer Thomas Zehnle said his client "placed his trust in the wrong person" by allowing Gates to handle his finances. Zehnle accused Gates of embezzling money from Manafort and asked the jury to assess Gates' credibility.

Some of the witnesses in the case appeared to bolster that argument. On Friday, accountant Cindy LaPorta, testifying under a grant of immunity, told prosecutors that it was Gates who directed Manafort's tax preparers to change loan amounts so that Manafort would be able to pay his 2014 tax bills.

She went along with it, though she said in court that "I very much regret" doing so.

But another witness, bookkeeper Heather Washkuhn, said that Manafort had approved "every penny" of the bills she paid for him. She and others said they were unfamiliar with the names of foreign companies that Manafort is alleged to have used to pay for lavish personal items, including "closets full" of ultra-expensive clothing, cars and landscaping worth millions of dollars.

Still, most experts say the government has no choice but to call its star witness. Brett Kappel, a campaign finance expert with law firm Akerman LLP, said Mueller's lawyers should not rest their case without calling Gates. He suggested that "the prosecution will put him on to demonstrate that he was a conduit for instructions from Manafort to the accountants and bookkeepers."

Gates could face nearly six years in prison, though the special counsel could ask the court to reduce his sentencing time depending on his willingness to cooperate with investigators.

Facing dozens of charges in Washington and Virginia federal court cases centered around his and Manafort's roles in an alleged scheme to hide foreign lobbying money from U.S. authorities, Gates struck the plea deal with Mueller's team in February.

Manafort, however, has pleaded not guilty to all charges brought against him by the special counsel. In a separate civil case, Manafort had accused the Justice Department and Mueller, who is tasked with investigating potential coordination between the Trump campaign and Russia, of exceeding their authority.

The trial resumed Monday afternoon. Judge T.S. Ellis has been pushing prosecutors to expedite their case and drop extraneous evidence. As a result, prosecutors said Wednesday that they were ahead of schedule, and expected to conclude their case in chief this week.

The most serious charges against Manafort are nine counts of bank fraud and conspiracy. If convicted, Manafort could be sentenced to as much as 30 years in prison for each count.