Mr. Manafort’s lawyers made the disclosure by accident, through a formatting error in a document filed to respond to charges that he had lied to prosecutors working for the special counsel, Robert S. Mueller III, after agreeing to cooperate with their investigation into Russian interference in the election.
The document also revealed that during the campaign, Mr. Manafort and his Russian associate, Konstantin V. Kilimnik, discussed a plan for peace in Ukraine. Throughout the campaign and the early days of the Trump administration, Russia and its allies were pushing various plans for Ukraine in the hope of gaining relief from American-led sanctions imposed after it annexed Crimea from Ukraine.
Prosecutors and the news media have already documented a string of encounters between Russian operatives and Trump campaign associates dating from the early months of Mr. Trump’s bid for the presidency, including the now-famous meeting at Trump Tower in Manhattan with a Russian lawyer promising damaging information on Hillary Clinton. The accidental disclosure appeared to some experts to be perhaps most damning of all.
“This is the closest thing we have seen to collusion,” Clint Watts, a senior fellow with the Foreign Policy Research Institute, said of the data-sharing. “The question now is, did the president know about it?”
The document gave no indication of whether Mr. Trump was aware of the data transfer or how Mr. Kilimnik might have used the information. But from March to August 2016, when Mr. Manafort worked for the Trump campaign, Russia was engaged in a full-fledged operation using social media, stolen emails and other tactics to boost Mr. Trump, attack Mrs. Clinton and play on divisive issues such as race and guns. Polling data could conceivably have helped Russia hone those messages and target audiences to help swing votes to Mr. Trump.
Both Mr. Manafort and Rick Gates, the deputy campaign manager, transferred the data to Mr. Kilimnik in the spring of 2016 as Mr. Trump clinched the Republican presidential nomination, according to a person knowledgeable about the situation. Most of the data was public, but some of it was developed by a private polling firm working for the campaign, according to the person.
Mr. Manafort asked Mr. Gates to tell Mr. Kilimnik to pass the data to two Ukrainian oligarchs, Serhiy Lyovochkin and Rinat Akhmetov, the person said. The oligarchs had financed Russian-aligned Ukrainian political parties that had hired Mr. Manafort as a political consultant.
A representative for Mr. Akhmetov said he did not request or receive any data from Mr. Kilimnik and has had no contact with either Mr. Kilimnik or Mr. Manafort since before the 2016 presidential campaign. Mr. Lyovochkin did not respond to a request for comment.
Why Mr. Manafort wanted them to see American polling data is unclear. He might have hoped that any proof that he was managing a winning candidate would help him collect money he claimed to be owed for his work on behalf of the Ukrainian parties.
About the same time, Mr. Manafort was also trying to curry favor with Oleg V. Deripaska, a Russian billionaire close to the Kremlin and an associate of Mr. Kilimnik. In July 2016, Mr. Manafort, then the Trump campaign chairman, told Mr. Kilimnik that he could offer Mr. Deripaska “private briefings,” according to emails reported by The Washington Post. Mr. Deripaska had claimed Mr. Manafort owed him millions from a failed business venture, and Mr. Manafort may have been trying to use his status in the campaign to hold him at bay.
The surprise disclosures about Mr. Manafort were the latest in two years of steady revelations about contacts between associates of Mr. Trump’s and Russian officials or operatives. In another development on Tuesday, the Russian lawyer who met with senior campaign officials at Trump Tower in June 2016 was charged with obstruction of justice in an unrelated case. Federal prosecutors in Manhattan said that the lawyer, Natalia V. Veselnitskaya, had pretended to a federal judge that she was purely a private defense lawyer when in fact she was working with the Russian government to thwart the civil prosecution of a Russian company.
Of the various Russian intermediaries to the Trump campaign, Mr. Kilimnik appears to be one of the most important to Mr. Mueller’s inquiry. A Russian citizen and resident, he faces charges from Mr. Mueller’s team of tampering with witnesses who had information about Mr. Manafort, but Mr. Kilimnik is not expected to ever stand trial. He did not respond to a phone call and email seeking comment on Tuesday.
His relationship with Mr. Manafort dates back years. The two men worked together to promote a Russia-aligned politician, Viktor F. Yanukovych, who rose to Ukraine’s presidency, was ousted in a popular uprising and fled to Russia in 2014. The two men continued working together over the next three years as Mr. Manafort’s financial troubles grew and investigators began to investigate a fraud scheme that eventually led to his conviction for 10 felonies.
In August 2016, apparently just before Mr. Manafort was fired from the Trump campaign, he and Mr. Kilimnik met to discuss a plan for Ukraine that seemed to further Russia’s interests. They also met several times afterward, including once in Madrid in early 2017. In an interview in February 2017 in Kiev, Ukraine’s capital, Mr. Kilimnik suggested the plan would have involved reviving the political fortunes of Mr. Yanukovych, the ousted Ukrainian leader.
For Russia, trying to influence the incoming Trump administration’s policy on Ukraine was of paramount importance. The economic sanctions imposed after Russia annexed Crimea damaged the Russian economy, and various emissaries have tried to convince administration officials to broker a resolution to a long-running guerrilla war between Russia and Ukraine.
In one previously reported case, for instance, an obscure Ukrainian lawmaker, Andrii V. Artemenko, worked with two associates of Mr. Trump’s, Felix Sater and Michael D. Cohen, to deliver one Ukraine “peace plan” to the White House. The plan landed on the desk of Michael T. Flynn, the president’s first national security adviser, just before he was forced to resign.
In an interview on Tuesday, Mr. Artemenko said that investigators working for Mr. Mueller had questioned him extensively about his efforts to promote that plan. Mr. Artemenko said he knew neither Mr. Manafort nor Mr. Kilimnik and stressed that his plan was different from theirs.
Prosecutors have accused Mr. Manafort of lying not only about his interactions with Mr. Kilimnik, but about other matters, including his contacts with senior administration officials in early 2017 and a payment related to a pro-Trump political action committee to cover his legal expenses. In November, after a dozen meetings with Mr. Manafort, they broke off the plea agreement with him, citing what they said was a series of lies.
In their filing, Mr. Manafort’s defense lawyers said that Mr. Manafort never intentionally misled federal authorities. Instead, they cast him as a sick man — troubled by “severe” gout, as well as by depression and anxiety — who made misstatements because of a faulty memory and lack of access to his own records.
They nevertheless said that they would not seek a hearing to challenge the prosecutors’ accusations that he lied, a decision that brings Mr. Manafort one step closer to being sentenced for his crimes.
The decision not to seek a hearing reflected the dearth of legal choices for Mr. Manafort, 69, who has been in jail in Northern Virginia since mid-June. One prosecution resulted in his jury conviction in August for bank fraud, tax fraud, and other financial crimes. A second led to his guilty plea in September on two conspiracy charges, including one stemming from a witness-tampering scheme with Mr. Kilimnik. Mr. Manafort faces at least 10 years in prison.
The plea agreement gives the prosecutors the power to almost unilaterally decide whether Mr. Manafort has violated it. Unless Mr. Manafort can show they acted in bad faith — a high bar — their judgment stands. The prosecutors could also decide to file new charges against Mr. Manafort for lying to them, but apparently, they do not plan to do so, according to Tuesday’s filing.
“They have him so deeply in the soup here that what both sides are almost saying is that this doesn’t matter,” said Nancy Gertner, a Harvard Law School professor.