Durham's star witness is Baker, who was the FBI's top lawyer in 2016 when he met with Sussmann about the Trump-Alfa tip. Sussmann's alleged crime stems from that meeting and whether he told Baker he wasn't representing any clients -- and was only there to help the FBI.

Over the years, Baker has given varying answers to this question, and may not have a clear recollection. Baker also has publicly defended the origins of the FBI's Russia probe, making him an awkward witness for Durham, who has pushed the idea that the investigation was improper. 

Prosecutors are armed with a text Sussmann sent to Baker one day before their meeting, where he allegedly wrote, "I'm coming on my own -- not on behalf of a client or company." This is not charged as a false statement, but Durham's team says it shows Sussmann's guilt.

"The text is strong circumstantial evidence," said Renzi, the criminal defense attorney. "Texts that people write, assuming they can be authenticated, are very powerful because they show someone's state of mind and can be directly attributed to them."

Prosecutors also plan to put former Clinton campaign manager Robby Mook on the stand, and an ex-staffer from Fusion GPS. They'll likely be asked about how the Clinton campaign worked with its attorneys and outside researchers to try to dig up dirt about Trump's ties to Russia, and how that material was peddled to reporters and to federal investigators. 

Sussmann's defense has highlighted internal notes from a March 2017 meeting of top Justice Department and FBI officials, in which one official allegedly said Sussmann had been representing a client or clients. The handwritten notes, turned over by Durham in recent weeks, appear to undercut prosecutors' central thesis that Sussmann was concealing his clients.

By the time of that March 2017 meeting, Sussmann's work as a cybersecurity lawyer for the Democrats and Clinton's campaign was well-known to the FBI, which had been investigating the Russian hack of Democratic National Committee servers and the dumping of internal Clinton campaign emails to undermine her campaign.

For his defense, Sussmann also plans to call witnesses from the FBI and Justice Department, including Justice Department Inspector General Michael Horowitz and Mary McCord, who was part of the DOJ's early oversight of the Russia probe. 

In his role as the Justice Department watchdog, Horowitz reviewed the origins of the Russia probe and concluded in a sweeping 2019 report that the inquiry was legitimately opened and wasn't motivated by anti-Trump bias.