A lawyer for Hillary Clinton's 2016 presidential campaign lied to the FBI to "manipulate" the agency in order to win the election, a court has heard.
Arguments in the trial of Michael Sussmann have begun with prosecutors saying he hoped to create an "October surprise" in the race's final weeks.
He is the first defendant to go on trial in an investigation into the FBI's original probe to find if Donald Trump was conspiring with Russia.
The charges arise from a meeting between Mr Sussmann, a cyber-security lawyer, and an FBI agent in 2016.
Mr Sussmann presented what he claimed was evidence of suspicious internet traffic connecting the Trump Organization to Russia's Alfa Bank. The FBI looked into the allegation and found nothing suspicious.
Prosecutors say Mr Sussmann lied by not disclosing in the meeting that he was working for the Democrat's campaign. The indictment charges that he claimed not to be representing any particular client and that he presented himself purely as a concerned citizen.
A lawyer for Mr Sussmann denounced the trial as an "injustice" at Tuesday's hearing in a Washington DC federal courthouse.
The Mueller inquiry concluded in 2019 that it could find no evidence of any criminal conspiracy between the Trump campaign and Russia, though it did determine Russia had sought to help Mr Trump win.
President Trump always said the Mueller inquiry was biased against him.
In court on Tuesday, prosecutor Brittain Shaw argued that Mr Sussmann had hoped to drop a bombshell in the final days of the November 2016 race.
"This case was about privilege," Ms Shaw said, describing the defendant as a "high-powered DC lawyer".
Lawyers for Mr Sussmann called the government's argument "nonsensical", arguing that the FBI would have known he had represented the Democratic National Committee earlier that year after their computer servers were hacked.
Also on Tuesday, prosecutors revealed what led the FBI to conclude that the evidence provided by Mr Sussmann did not prove any Russian collusion.
The server at the Trump Organization uncovered by Mr Sussmann "was merely a spam email server used for sending out marketing emails", Ms Shaw told the jury.
"The server did not reflect a crime, nor was it a threat to national security," she added.
Two other people have been charged with crimes as part of Mr Durham's probe. One pleaded guilty in 2020 to altering evidence that was used to secure an FBI wiretap on Trump aide Carter Page.
In the other case, a Russia analyst has been charged with lying to the FBI about his sources.
Though there is no evidence that Mrs Clinton herself directed the alleged Sussmann smear, political opponents have poured scorn on her remarks this month warning about the dangers of disinformation and conspiracy theories.