For Manafort, though, the charges come at a perilous time, just hours before his attorneys filed a legal brief arguing that he should be allowed to be freed from home confinement on bond pending his trial, scheduled for next month in Alexandria, Va. He faces a second trial in Washington in September.

Special counsel Robert S. Mueller III on Capitol Hill last June. (Andrew Harnik/AP)

Prosecutors filed court papers Monday accusing Manafort and Kilimnik of attempting to sway the testimony of two potential witnesses who might offer evidence against Manafort. Authorities charge that the conduct of Manafort and Kilimnik amounts to witness tampering and have asked a judge to revise or revoke Manafort’s bail package.

Neither Kilimnik nor a Manafort spokesman responded to messages seeking comment.

But in a biting, nine-page filing Friday evening, Manafort’s attorneys denied the allegations and accused special counsel Robert S. Mueller III’s team of conjuring up “a sinister plot” from a scant record of telephone calls and de-encrypted texts. They said the special counsel’s latest “very public and very specious” filing was a pressure tactic on Manafort that may have “irreparably damaged” his right to a fair trial in the District of Columbia, signaling they may seek to move the case to Virginia.

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Still, the new counts could make it harder for Manafort to avoid jail before he goes on trial for alleged financial crimes that largely predate his time on the Trump campaign.

“Going to the grand jury for a superseding indictment underlines how serious the government is about Manafort’s conduct,’’ said Joyce Vance, a U.S. attorney in Alabama during the Obama administration. “A defendant in a drug case would have his release revoked and be sent to jail pending trial for violating the terms of his release like this.”

The new charges revolve around allegations that Manafort, 69, and Kilimnik, 47, tried to influence two public-relations executives who were involved in lobbying work in 2012 on behalf of the Ukraine government.

According to court filings, after Manafort was accused in February of additional instances of secretly lobbying on behalf of a foreign government without registering as a lobbyist, he and Kilimnik repeatedly contacted the two executives to emphasize that their past work together did not involve American officials and therefore did not constitute lobbying in the United States. The lead executive told investigators he knew that claim to be untrue.

Manafort allegedly sent one of the executives a text that said: “We should talk. I have made clear that they worked in Europe.”

Kilimnik, according to court papers, wrote a message to the other executive two days later saying: “Basically P wants to give him a quick summary that he says to everybody (which is true) that our friends never lobbied in the US, and the purpose of the program was EU.”

Prosecutors say that the “P” in the message refers to Manafort and that the assertions in the texts are untrue.

Manafort’s attorneys countered that prosecutors presented “scant proof” — mostly “irrelevant, innocuous” texts, and one completed phone call of five attempts that lasted one minute, 24 seconds. “Closer scrutiny of this ‘evidence’ reveals that the Special Counsel’s allegations are without merit because Mr. Manafort’s limited communications cannot be fairly read, either factually or legally, to reflect an intent to corruptly influence a trial witness.”

Manafort lawyers said even the messages cited by prosecutors “are entirely consistent with Mr. Manafort’s stated position and repeated assertion of his innocence,” that his lobbying was “European-focused” and that he had no idea the two recipients would be called as witnesses.

Mueller’s office filed court papers Monday accusing Manafort of witness tampering. Manafort had been close to getting court approval for a $10 million bail deal that would have freed him from home detention. He has been charged with money laundering, tax and bank fraud, and conspiracy in two federal cases as part of Mueller’s ongoing probe. He has pleaded not guilty in both cases.

Sending Manafort to jail ahead of his trial would intensify the pressure on him to reach a plea deal with prosecutors. In white-collar investigations, agents and prosecutors often try to turn defendants into witnesses against other suspects, “flipping” those people to climb higher up the organization. Manafort has repeatedly declared his intention to fight the charges in both jurisdictions.

The alleged coverup stems from work Manafort, Kilimnik and others did with former European politicians, referred to informally as the “Hapsburg group,” to advocate on Ukraine’s behalf to U.S. and European officials.

The indictment charges that former Ukraine president Viktor Yanukovych and his political party retained lobbyists through Manafort to serve as intermediaries with the former European politicians. The politicians lobbied members of Congress and executive-branch officials on Ukraine in coordination with Manafort and his longtime deputy and former co-defendant, Rick Gates, according to prosecutors. Gates pleaded guilty to conspiracy and lying to investigators earlier this year in a cooperation deal with the government.

The FBI has assessed that Kilimnik had ongoing ties to Russian intelligence, including during the 2016 presidential campaign when Manafort and Gates were in contact with him, prosecutors have said in court filings. Gates told investigators that he understood Kilimnik to be a former officer with the Russian military intelligence service, and others who worked with Manafort in Ukraine said that Kilimnik did not hide his intelligence background. Kilimnik has denied such ties.

Manafort has acknowledged staying in frequent contact with Kilimnik during the time he worked for Trump’s campaign. He has said he met with Kilimnik in person in May 2016 and again in August 2016 in New York City, about two weeks before Manafort resigned as Trump’s campaign chairman.

Manafort hired Kilimnik, who is a Russian national, as a translator in his office in Kiev, the Ukrainian capital, in 2005. Kilimnik later took on more responsibilities, serving as Manafort’s liaison to Oleg Deripaska, a Russian billionaire who paid Manafort to advise him on investments. Kilimnik also became the manager and in-country representative for Manafort’s Ukrainian political consulting business.

The two remained close while Manafort was working for Trump’s campaign. Kilimnik told The Washington Post they emailed and spoke frequently.