Captured Ukrainian Oligarch Was Figure in Russian Election Meddling Investigation

His name had surfaced as an influential figure in Ukraine with potential inside knowledge of Russian electoral meddling in the United States, though for years he had steadfastly denied it.

But in recent days, the ground has shifted dramatically under Viktor Medvedchuk, a Ukrainian politician who is a close confidant of President Vladimir V. Putin of Russia and who had also been a client of the Republican political consultant Paul J. Manafort.

Mr. Medvedchuk went into hiding early in the war, Ukrainian officials say, and was detained this week. President Volodymyr Zelensky posted on Tuesday a picture on Telegram of the politician, looking tired and disheveled, wearing handcuffs. He was arrested after violating terms of his house arrest while awaiting trial for treason, in a case opened last year.

That case is related to coal trading with pro-Russian separatists, but more broadly it has to do with the swirl of financial and political intrigue surrounding Moscow’s operations to influence politics in foreign countries.

For now, it’s unclear whether Mr. Medvedchuk will ever testify in court in Ukraine or be interviewed by investigators looking into Russian influence operations elsewhere. Mr. Zelensky said he would seek to trade Mr. Medvedchuk to Russia for Ukrainian prisoners of war.

“I offer the Russian Federation to trade your man for our boys and girls now in captivity,” Mr. Zelensky said. “It’s important our law enforcement and military study such a possibility.”

A trade would presumably put Mr. Medvedchuk in Russia, out of reach of researchers tracking Russian attempts to influence political outcomes abroad, in which Mr. Medvedchuk is said to have played a central role in Ukraine.

His relevance to Russian electoral meddling in the United States related to his ties to Mr. Manafort, and he was not described as playing a central role in a special prosecutor’s report or in two federal trials of Mr. Manafort.

Still, Mr. Medvedchuk has been close both politically and personally to Mr. Putin for more than two decades, and he was a prominent figure in the pro-Russian wing of Ukrainian politics, a circle where Mr. Manafort found several clients.

Mr. Putin is the godfather to Mr. Medvedchuk’s daughter. The two men met frequently over the years, and Russian air traffic control authorities granted special exemptions for Mr. Medvedchuk’s private jet on flights to Moscow, he said in an interview in 2017.

Some European politicians, including the former chancellor of Germany, Angela Merkel, had publicly endorsed a role for Mr. Medvedchuk as an intermediary in the standoff between Russia and Ukraine, given his personal ties to Mr. Putin.

But in Ukraine, outside of a narrow base of support mostly in the country’s east, he was widely viewed as a loathsome quisling who had reaped wealth from energy deals with the Kremlin while promoting Russian foreign policy goals, including weakening the central government under a federalization overhaul that he had championed for years.

At various times, he had served as deputy speaker of Parliament, a presidential adviser and a negotiator in prisoner exchanges with Russia. And as a figure at the nexus of various financial and political influence operations run by the Kremlin, Mr. Medvedchuk’s importance extended beyond Ukraine.

Mr. Manafort, before he became chairman of Donald J. Trump’s presidential campaign in 2016, worked for a decade as a consultant for Russian-leaning politicians in Ukraine, including the Opposition Bloc party, in which Mr. Medvedchuk was one of three leading figures.

Mr. Manafort advised the party on its electoral strategy based on polling, Mr. Medvedchuk said in the interview in Kyiv in 2017. He recalled a party conference that the American consultant had attended ahead of parliamentary elections a few years earlier. Mr. Manafort, he said, had endorsed the party’s pro-Russia policies as electorally sound in Ukraine’s southeastern region.

“The principles, including those Paul Manafort presented, said we should stick to positions we held on economic integration with Russia,” he said. He added that Mr. Manafort had “supported these ideas” to win votes in Russian-speaking areas where the Opposition Bloc was popular.

Mr. Manafort resigned from the Trump campaign in August 2016, after the Ukrainian authorities released entries from an accounting document showing he had received $12.7 million from pro-Russian politicians in Ukraine under a payment scheme under investigation for possibly being illegal.

As the investigation into Russian meddling ramped up in the United States in 2017, Mr. Medvedchuk’s name surfaced as one of several pro-Russian Ukrainian politicians close to both Mr. Manafort and Mr. Putin. That year, Reuters reported that F.B.I. investigators were examining 18 phone calls and text messages between people close to Mr. Trump and people with ties to Mr. Putin, including Mr. Medvedchuk.

At the time, Mr. Medvedchuk denied wrongdoing. It is unclear whether any potential criminal prosecution now in Ukraine — if he is not traded in a prisoner swap — would touch on these issues or focus only on his role in Ukrainian politics.

Mr. Medvedchuk never downplayed what he described as a “personal” relationship of friendship with Mr. Putin, despite the two countries being in a military conflict since 2014. In the interview, he said that in 2004 he had asked Mr. Putin to become the godfather of his daughter and that Mr. Putin “with pleasure agreed.”

After his capture, Mr. Zelensky posted a photograph on Telegram showing Mr. Medvedchuk in handcuffs and wearing a Ukrainian military uniform. Mr. Zelensky said he had tried to disguise himself as a soldier and for this reason would be subject to military law and could be offered in exchange for Ukrainian prisoners of war.

The Kremlin spokesman, Dmitri S. Peskov, offered no immediate response to the arrest of the father of Mr. Putin’s goddaughter, saying he could not confirm that Mr. Medvedchuk had been detained, according to Russian state media.