Donald Trump’s lawyer Rudy Giuliani has warned Robert Mueller, the special counsel investigating possible collusion between Russia and the Trump campaign during the 2016 presidential election, that the White House is close to refusing to grant an interview with the president.

Giuliani took the increasingly belligerent tone of the White House up a notch on Sunday when he called the Russian investigation the “most corrupt I’ve ever seen”. Speaking on This Week on ABC News, he accused the special counsel of assembling a team of investigators around him that included “very, very severe partisans working on an investigation that should have been done by people who are politically neutral”.

Asked whether they had made a decision on whether or not Trump should participate in an interview with the inquiry, he replied: “We have not determined he will not sit down with Mueller, but we are close to that.”

Giuliani’s round of the Sunday TV political talkshows is the latest sign that the core Trump team has decided to abandon its earlier approach of being seen to cooperate with the Russia investigation, and move towards an antagonistic position. On Friday, Giuliani told the New York Times that Mueller would get his interview with the president only if he could satisfy the White House that he had evidence that Trump had committed a crime.

The attorney and former mayor of New York, who is a long-standing friend of Trump’s, walked back that suggestion a little on Sunday. He said the White House did not require evidence of a criminal deed but at least some factual basis supporting suspicion of a crime.

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Giuliani revealed to CNN’s State of the Union that the White House legal team had debriefed all the witnesses to the Mueller investigation, and reviewed 1.4m pages of documents handed over to the special counsel. As a result, he claimed, he could confidently say that Trump had nothing to answer.

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“I have a pretty good idea because I’ve seen all the documents they have, we’ve debriefed all their witnesses. They have nothing. They would not be pressing for this interview if they had anything.”

He also professed to be untroubled by either the prospect of Trump being served with a subpoena to appear before the inquiry, or by his former lawyer, Michael Cohen, flipping and handing over all his information to Mueller. “To subpoena the president has never been done successfully in the history of this country, and there is very strong law saying the president can’t be subpoenaed,” he said.

On Cohen, Giuliani said “I don’t know what he has to flip over. I do know there’s no evidence of wrongdoing with President Trump. So we are very comfortable. If he believes it’s in his best interests to cooperate, God bless him.”