Lev Parnas and his business partner, Igor Fruman, were at Dulles International Airport near Washington two years ago, holding one-way tickets for a Lufthansa flight to Frankfurt, when the F.B.I. caught up with them.
The two Soviet-born businessmen, who had climbed to prominent places in the orbit of President Donald J. Trump, were arrested before boarding the flight and charged with several campaign finance violations, including conspiring to funnel a Russian tycoon’s money into American politics.
In the years that followed, Mr. Parnas began to open up about his role in events connected to Mr. Trump’s first impeachment. He provided material to House investigators and acknowledged participating in an effort by Mr. Trump’s personal lawyer, Rudolph W. Giuliani, to pressure Ukrainian officials to investigate Joe Biden, a leading Democratic presidential candidate at the time who subsequently beat Mr. Trump.
Still, the campaign finance charges loomed. Even as Mr. Fruman and a second man charged in the case, David Correia, pleaded guilty, Mr. Parnas and a fourth defendant, Andrey Kukushkin, maintained their innocence. Last week, the two men went to trial in Federal District Court in Manhattan.
On Friday, his activity as a political donor caught up with Mr. Parnas as a jury convicted him of all the charges against him. Mr. Kukushkin was also convicted.
That verdict came after about six hours of deliberations, following testimony from, among others, a onetime finance director of a super PAC supporting Mr. Trump; an ex-chief of staff to a former Republican congressman from Texas; and the 2018 Republican candidate for governor of Nevada.
Prosecutors outlined a two-part scheme, saying Mr. Parnas had lied to the Federal Election Commission about certain campaign donations he had made and that he and Mr. Kukushkin had tried to gain favor among candidates with contributions tied to the Russian tycoon, Andrey Muraviev, with whom they were involved in a cannabis business.
“The purpose behind this conspiracy was influence buying,” a prosecutor, Hagan Cordell Scotten, said during closing arguments on Thursday, adding: “The voters would never know whose money was pouring into our elections.”
Throughout the trial, Mr. Parnas’s lawyer, Joseph A. Bondy, presented his client as an enterprising businessman who was a longtime proponent of legalizing marijuana and who also wanted to lead a company that would bring natural gas from the United States to Europe to help prevent a Russian “stranglehold” on energy.
Although Mr. Parnas had strayed beyond his area of expertise, Mr. Bondy said, he had not “willfully” broken any laws. Instead, Mr. Bondy argued, Mr. Parnas had been flummoxed by the intricacies of campaign finance rules.
“He was in well over his head,” Mr. Bondy said. “But he had some good ideas.”
Gerald B. Lefcourt, a lawyer for Mr. Kukushkin, said that his client, too, had lacked “criminal intent” and was not involved in making political donations. Mr. Lefcourt said Mr. Kukushkin had planned to start a legal cannabis business and believed that Mr. Parnas and Mr. Fruman were politically connected experts who could help obtain the necessary licenses.
Instead, Mr. Lefcourt said, the two had taken money meant for business expenses and used it to pay off debts and finance their lavish lifestyles, essentially swindling Mr. Kukushkin.
“They looked at him as unsophisticated and inexperienced — a rube,” Mr. Lefcourt said during his closing argument.
The charges were based largely on extensive documentation that included campaign contribution receipts, bank records and logs of WhatsApp messages exchanged by the main figures in the case.
The evidence helped fill out a picture of Mr. Parnas, who was born in Ukraine, grew up with humble roots in Brooklyn, worked his way into an upper echelon of Republican donors and became a key figure in the impeachment of an American president.
Prosecutors said records showed that a $325,000 donation to the super PAC backing Mr. Trump, America First Action, Inc. had been falsely reported as coming from Global Energy Producers, a company started by Mr. Parnas and Mr. Fruman. In reality, the prosecutors said, the money came from a loan Mr. Fruman had taken out on a condo he owned.
In court papers, prosecutors said the purpose of the donation was to present Global Energy as a successful company and “obtain access to exclusive political events and gain influence with politicians.”
Prosecutors also introduced financial records that they said showed that donations to the Nevada governor candidate, Adam Laxalt, and Nevada Republican, Wesley Duncan, had been made with a credit card tied to a New York company controlled by Mr. Fruman’s brother.
Records also showed that in fall 2018, two companies owned by Mr. Muraviev had wired $500,000 apiece to the company.
The defense lawyers challenged the idea that the donations could be tied directly to Mr. Muraviev. Under cross-examination by Mr. Bondy, a forensic accountant working for the F.B.I. acknowledged that her “initial assessment” had been that Mr. Muraviev was not the source of the Nevada donations.
Minutes later, one of Mr. Kukushkin’s lawyers, Faith Alison Friedman, asked the accountant: “As you sit here today, you are unable to say definitively” what money went to pay those donations?
“That’s correct,” replied the accountant, Kimberly Espinoza.
But prosecutors pointed to several text message exchanges that they said indicated otherwise.
Among them was a message from Mr. Parnas’s assistant, Deanna Van Rensburg, who wrote to Mr. Kukushkin asking when “Adam Laxalt’s team” could expect two checks. Mr. Kukushkin wrote back, saying: “the money was transferred ~ $1M to Lev & Igor’s company a long time ago to cover all the contributions as planned.”
Mr. Fruman replied with apparent exasperation, writing: “Andrey, what kind of mushroom did you eat today!?” “What are these discussions with our secretary???????”
The next day Mr. Parnas wrote to Mr. Fruman, Mr. Kukushkin and Mr. Muraviev: “I don’t want to discuss everything over text.”
That exchange was part of a trove of messages, some in Russian, that was introduced into evidence and showed communications between Mr. Parnas, Mr. Fruman, Mr. Kukushkin, Mr. Muraviev and, occasionally, others.
The emoji-laden exchanges showed the four men discussing payments to Mr. Fruman’s brother’s company, plans to attend political rallies, and “licensing in the key states,” an apparent reference to the cannabis business, and occasionally squabbling over money.
Mr. Parnas and Mr. Fruman shared pictures of themselves with political figures, including Mr. Trump, Mr. Giuliani and Ron DeSantis, the Republican candidate for governor in Florida at the time. The pictures seemed designed to present the two as being close to Mr. Trump as a way of convincing Mr. Kukushkin and Mr. Muraviev to finance their political activities.
After Mr. Kukushkin wrote that “our contributions are kind of way too much,” Mr. Parnas responded with a photo that showed him smiling broadly and standing with Ivanka Trump and Jared Kushner.
Mr. Parnas had been an ardent supporter of Mr. Trump’s, sometimes showing up at rallies in a “Make America Great Again” hat. While working with Mr. Giuliani in 2018 and 2019, Mr. Parnas traveled to Kyiv to press officials there to investigate Mr. Biden’s son Hunter. He was also in regular touch with Yuriy Lutsenko, who, as Ukraine’s chief prosecutor at the time, was urging the removal of the U.S. ambassador in Kyiv.
Mr. Trump’s eventual decision to recall the ambassador, Marie L. Yovanovitch, was cited during his first impeachment and became central to a federal investigation into whether Mr. Giuliani broke lobbying laws.
By the time the trial began, Mr. Parnas had long soured on Mr. Giuliani and Mr. Trump, saying he regretted ever trusting them.
In addition to providing material to the House Intelligence Committee as part of its impeachment inquiry, he made public a phone video from a small dinner in early 2018 at the Trump Hotel in Washington at which Mr. Trump, referring to Ms. Yovanovitch, said “get rid of her.”
After the arrest at the airport, Mr. Trump told reporters he did not know Mr. Parnas.