PROSECUTORS working for special counsel Robert Mueller are accusing former Trump campaign chairman Paul Manafort of trying to tamper with witnesses in his ongoing criminal cases.

Mueller’s team says in a new court filing that Manafort and one of his associates “repeatedly” contacted two witnesses in an effort to influence their testimony. The contacts allegedly occurred while Manafort was under house arrest.

The two witnesses are not named in court filings. But prosecutors say they worked with Manafort in organizing a group of former European officials who lobbied within the U.S. without registering.

Mueller’s team is asking a federal judge to hold a hearing to decide whether to revoke Manafort’s release. Manafort faces several felony charges in two federal cases. He has pleaded not guilty.

The claims came after Donald Trump has asserted his presidential power and escalated his efforts to discredit the special counsel Russia probe, declaring he has the “absolute right” to pardon himself and attacking the investigation as “totally UNCONSTITUTIONAL!”

Mr Trump’s comments on Twitter came a day after lawyer Rudy Giuliani played down the possibility that the president could pardon himself, suggesting he might have that authority but would be unwise to use it.

“Pardoning himself would be unthinkable and probably lead to immediate impeachment,” Mr Giuliani, a member of Mr Trump’s legal team, told NBC’s Meet the Press on Sunday.

“And he has no need to do it, he’s done nothing wrong.”

As has been stated by numerous legal scholars, I have the absolute right to PARDON myself, but why would I do that when I have done nothing wrong? In the meantime, the never ending Witch Hunt, led by 13 very Angry and Conflicted Democrats (& others) continues into the mid-terms! — Donald J. Trump (@realDonaldTrump) June 4, 2018

Mr Trump’s legal team is making clear that it will combat any effort to force the president to testify in front of a grand jury.

MORE: Meeting time set for Trump, Kim

The appointment of the Special Counsel is totally UNCONSTITUTIONAL! Despite that, we play the game because I, unlike the Democrats, have done nothing wrong! — Donald J. Trump (@realDonaldTrump) June 4, 2018

Mr Giuliani underscored one of the main arguments in a newly unveiled letter sent by the president’s lawyers to Mr Mueller back in January: A president can’t be given a grand jury subpoena as part of the investigation into foreign meddling in the 2016 election.

But Mr Giuliani, in a series of television interviews, broke with one of their bolder arguments in the letter that a president could not have committed obstruction of justice because he has ultimate authority over any federal investigation.

Yet the former New York City mayor, who was not on the legal team when the letter was written, added that Trump “probably does” have the power to pardon himself, an assertion challenged by legal scholars.

He says the president’s legal team hasn’t discussed that option, which many observers believe could plunge the nation into a constitutional crisis.

“I think the political ramifications would be tough,” Mr Giuliani told ABC’s This Week. “Pardoning other people is one thing, pardoning yourself is tough.”

Mr Trump has issued two unrelated pardons in recent days and discussed others, a move that has been interpreted as a possible signal to allies ensnared in the Russia probe.

The letter is dated 29 January and is addressed to Mr Mueller from John Dowd, a Trump lawyer who has since resigned from the legal team.

Mr Mueller has requested an interview with the president to determine whether he had criminal intent to obstruct the investigation into his associates’ possible links to Russia’s election interference.

Mr Giuliani said that a decision about an interview would not be made until after the president’s summit with North Korean leader Kim Jong-un on June 12 in Singapore, and he cast doubt that it would occur at all.

“I mean, we’re leaning toward not,” Mr Giuliani told ABC.

“But look, if they can convince us that it will be brief, it would be to the point, there were five or six points they have to clarify, and with that, we can get this — this long nightmare for the — for the American public over.”

In addition to the legal battles, Mr Trump’s team and allies have waged a public-relations campaign against Mr Mueller and the Justice Department to discredit the investigation and soften the impact of the special counsel’s potential findings.

Mr Giuliani said last week that the special counsel probe may be an “entirely illegitimate investigation” and need to be curtailed because, in his estimation, it was based on inappropriately obtained information from an informant and Comey’s memos.

In reality, the FBI began a counterintelligence investigation in July 2016 to determine if Trump campaign associates were co-ordinating with Russia to tip the election.

The investigation was opened after the hacking of Democratic emails that intelligence officials later formally attributed to Russia.

Mr Trump’s team has requested a briefing about the informant, but Mr Giuliani said that the president would not order the Justice Department to comply because it would negatively affect public opinion.

But he continued to cast doubt on the special counsel’s eventual findings, suggesting that Mr Trump has already offered explanations for the matters being investigated and that the special counsel was biased against the president.

“For every one of these things he did, we can write out five reasons why he did it,” Mr Giuliani said.

“If four of them are completely innocent and one of them is your assumption that it’s a guilty motive, which (Trump) would deny, you can’t possibly prosecute him.”

Mr Trump’s legal team has long pushed the special counsel to narrow the scope of its interview.

Mr Giuliani also suggested that Mr Trump’s lawyers had been incorrect when they denied that the president was involved with the letter that offered an explanation for Donald Trump Jr.’s 2016 Trump Tower meeting with Russians who offered damaging information on Democrat Hillary Clinton.

“This is the reason you don’t let the president testify,” Mr Giuliani told ABC.

“Our recollection keeps changing, or we’re not even asked a question and somebody makes an assumption.”

If Mr Trump does not consent to an interview, Mr Mueller will have to decide whether to go forward with a historic grand jury subpoena. His team raised the possibility of subpoenaing the president in March, but it is not clear if it is still under active consideration.

A court battle is likely if Mr Trump’s team argues that the president can’t be forced to answer questions or be charged with obstruction of justice.

President Bill Clinton was charged with obstruction in 1998 by the House of Representatives as part of his impeachment trial. And one of the articles of impeachment prepared against President Richard Nixon in 1974 was for obstruction.