Former Trump campaign chairman Paul Manafort, seen leaving federal court in Washington in November. (Andrew Harnik/AP)

Former Trump presidential campaign chairman Paul Manafort plans to fight prosecutors’ claims that he tried to tamper with a witness — an accusation that could get him sent to jail before he goes on trial this summer on conspiracy and money laundering charges.

U.S. District Judge Amy Berman Jackson of the District of Columbia set a June 15 hearing to weigh prosecutors’ demand that she revoke or tighten the terms of Manafort’s release while he is pending trial. Manafort, 69, has been on home detention.

The judge gave Manafort’s lawyers until Friday to present a written rebuttal to accusations by prosecutors that he and a longtime associate repeatedly contacted two executives at a public relations firm in hopes of persuading them to provide false testimony about secret lobbying they did at Manafort’s behest in 2013.

“Mr. Manafort is innocent, and nothing about this latest allegation changes our defense,” Manafort spokesman Jason Maloni said in a statement. “We will do our talking in court.”

In a Monday night filing, prosecutors with special counsel Robert S. Mueller III’s office asked the judge to revoke or revise the conditions of Manafort’s bail because they had found probable cause to believe he sought to tamper with witnesses.

In their filing, prosecutors said that Manafort and a longtime associate reached out to two unidentified potential witnesses in February, shortly after new charges were filed accusing Manafort of failing to register as a foreign agent while lobbying on behalf of a foreign government. Those two people, prosecutors said, had worked with Manafort on that lobbying in 2012 and 2013.

Manafort called and texted one of those people repeatedly over a four-day period, according to phone and data storage records reviewed by the FBI.

Prosecutors say it appears Manafort and his longtime associate — identified by people close to the case as Konstantin Kilimnik — were seeking to get the two people to say, if investigators asked, that their past work together was focused on European officials and did not involve lobbying U.S. officials.

“We should talk,” Manafort allegedly texted in February to a person identified only as “D1” in court papers. “I have made clear that they worked in Europe.”

Days later, court papers say the person identified as Kilimnik texted the other individual, writing, “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 the “P” was Paul Manafort. Kilimnik did not respond to messages seeking comment.

The FBI said one of the public relations firm’s executives, who also is not named in the filing, told the government they “understood Manafort’s outreach to be an effort to ‘suborn perjury’ ” by encouraging others to lie to federal investigators by concealing the firm’s U.S. work.

If a judge agrees, Manafort could have his bail revoked and be forced to await trial in jail.

He goes on trial on tax and bank fraud charges in July in Alexandria, Va.; the trial in the District on the lobbying charges is to begin in September.

Sending Manafort to jail before trial could intensify the pressure on him to reach a plea deal with prosecutors.

Patrick Cotter, a former federal prosecutor who is now in private practice in Chicago, said that if the government’s claims against Manafort are true, he is in fresh trouble.

“What’s unusual, what leaps out at me, is the degree of specificity in the allegation. Add the fact that the intended recipients of the message have stated that they believed they were being asked to lie in matters directly related to the investigation, and it suggests this is a very serious allegation against Mr. Manafort,” Cotter said.

The court filing indicates that some of the suspicious messages were recovered from cloud storage of Manafort’s phone, while others were retrieved from the phones of the two individuals, who have spoken to investigators about the contacts.

At issue is a collection of former senior European officials, informally referred to as “the Hapsburg group,” who were secretly retained in 2012 by Manafort as part of his lobbying work for Ukraine, according to prosecutors. Manafort and PR executives steered those former European officials to meetings with U.S. politicians, trying to sway the Americans to be more favorable toward the Ukrainian government, according to prosecutors.

The work preceded Manafort’s role with the Trump campaign from March to August of 2016.

The government filing comes at a key time in Manafort’s case, amid a window for any potential, last-minute plea negotiations before he is to face trials.

The filing also comes amid new signs of pressure on Manafort’s financial situation. Unnamed supporters last week announced the creation of a new legal defense fund for him. The fund’s website was initially registered in December by Bruce E. Baldinger, a New Jersey real estate lawyer who has worked for years with Manafort, according to a person familiar with the situation.