Special Counsel David Weiss and his prosecutors plan to argue at Hunter Biden’s tax evasion trial next month that the first son was improperly lobbying on behalf of a Romanian businessman who was under investigation in his home country — an allegation critical to the government’s case that Biden was making millions of dollars and willfully avoided taxes that he was able to afford. The government says it plans to call a Biden business associate who will testify about the arrangement.

The calling of witnesses to testify about improper lobbying by Biden could indicate that he may face charges down the road for violating the Foreign Agent Registration Act. Mr. Weiss has long been believed to be investigating Biden’s overseas business dealings, during which he traded on his father’s name to reap millions of dollars in consulting income.

In a court filing to a federal judge at Los Angeles, Mr. Weiss’s top prosecutors — Leo Wise and Derek Hines — said they plan to call “Business Associate 1,” who worked with Biden on foreign lobbying and consulting efforts. Biden and this associate “entered into an oral agreement … purportedly to help a Romanian businessperson … contest bribery charges he was facing in Romania.” The Romanian is simply identified as “G.P.” in the court filing. 

“G.P. was a Romanian businessman who was under criminal investigation in Romania,” the prosecutors write. “G.P. sought to retain Business Associate 1, the defendant, and Business Associate 2, to attempt to influence U.S. government agencies to investigate the Romanian criminal investigation of G.P, and thereby cause an end to the investigation of G.P. in Romania.”

Messrs. Wise and Hines — who successfully won a conviction of Biden at a trial related to the first son’s purchase of a gun while addicted to crack cocaine —  allege in their filing that Biden and his business associates were “concerned” about the potential political ramifications for President Biden, and therefore sought to keep it from the public. 

Prosecutors allege that “Business Associate 1” improperly set up a shell company to “provide management services to real estate properties in Romania.” In reality, they say, the entity was used to pay more than $3 million to the three American business associates — money that was split evenly between Biden and his two partners. 

Messrs. Wise and Hines say in a footnote that they plan “to file an exhibit under seal which is a transcript of statements by Business Associate 1, and features some of the questions the government may ask at trial.”

The tale of Biden’s work for a Ukrainian energy company, Burisma, and a private equity firm in Communist China will also be introduced at trial, though the government is not expected to  argue that those two jobs involved improper lobbying work. 

The disclosure about the work for the Romanian businessman and the testimony of Biden’s business associate was made in response to a motion by the first son’s attorneys to exclude mention of his foreign dealings during the trial. Biden’s attorneys asked that allegations of improper work abroad be excluded, which led Messrs. Wise and Hines to shoot back that they would, instead of just raising allegations, produce evidence of improper work at trial. 

“The government does not intend to reference allegations at trial. Rather, the government will introduce the evidence described above, including that the defendant and Business Associate 1 received compensation from a foreign principal who was attempting to influence U.S. policy and public opinion and cause the United States to investigate the Romanian investigation of G.P in Romania,” the prosecution writes. 

Beyond “Business Associate 1,” Messrs. Wise and Hines have offered immunity to Biden’s sister-in-law and ex-girlfriend, Hallie Biden, and her sister, Elizabeth Secundy, who had a close relationship with Hunter Biden, in exchange for the two women’s testimony. Ms. Biden, who is the widow of Beau Biden, was dating the first son around the time he was addicted to drugs, working for foreign interests, and allegedly dodging his taxes. She testified at the Delaware gun trial where she detailed her brother-in-law’s erratic behavior and his crippling drug addiction. 

Mr. Weiss and his deputies have claimed they have a treasure trove of evidence that will prove Biden’s guilt in avoiding more than $1 million in taxes. In an evidence list that was filed with the court on July 31, Mr. Weiss listed more than 250 pieces of evidence such as emails, text messages, purchase histories, and bank statements.

Among the evidence listed are thousands of dollars in tuition sent by Biden to Columbia University for his daughter Naomi’s law school tuition, credit card payments, and a $10,000 membership fee at a country club. Biden’s Venmo transactions — presumably showing large payments at a time when he was failing to pay his taxes — will also be included. 

Biden was indicted by Mr. Weiss for the alleged tax crimes in December. According to that document, Biden’ made a ‘s gross income was nearly $8 million in those four years. He spent more than half of that money on personal expenses, including more than $1.5 million in cash withdrawals at a time when he was crippled by a severe drug addiction.

He also spent nearly $700,000 on payments to “various women,” $400,000 on clothing and accessories, and $300,000 on his children’s tuition. Biden dispersed more than $70,000 for rehabilitation and nearly $200,000 on “adult entertainment.”

The trial is set to begin at Los Angeles on September 9. If convicted, Biden faces nearly 20 years in prison and a hefty fine.