Rudy Giuliani, the man leading President Donald Trump's legal team in the Russia investigation, drew a new line in the sand on Friday as the two sides battle over whether Trump will testify under oath.

Giuliani said he wants special counsel Robert Mueller to show any evidence he may have that Trump committed a crime, and prove that Trump's testimony is essential in order for Mueller to complete his investigation.

Mueller and his prosecutors are looking into Russian meddling in the 2016 US election and whether members of the Trump campaign cooperated with Russian operatives. Trump is also the subject of an obstruction-of-justice inquiry related to his actions after he took office.

The president's legal team had been cooperating with Mueller early on, but changed its posture after a major shakeup.

Rudy Giuliani, President Donald Trump's lead defense attorney in the Russia investigation, set new terms for his client's cooperation in the Russia probe.

Before Trump agrees to testify under oath, Giuliani says he wants special counsel Robert Mueller to show evidence that Trump committed a crime, and that the president's testimony is essential for Mueller to complete his investigation.

The new stance reflects a shifting legal strategy in Trump's camp, The New York Times reported on Friday night. The president's lawyers had been cooperating with Mueller behind the scenes, despite Trump's frequent public broadsides against the special counsel and the career prosecutors investigating Russian meddling in the 2016 US election.

As part of that inquiry, investigators are looking into whether members of the Trump campaign colluded with the Russians. Separately, Trump is also the focus of an obstruction-of-justice probe related to his actions while in office.

Trump has not formally been accused of a crime, and he has denied any wrongdoing.

According to The Times, Giuliani wants Mueller to explain why the Justice Department authorized him to launch an obstruction-of-justice inquiry into Trump when the special counsel was originally appointed to investigate Russian interference.

The May 2017 order that established Mueller as special counsel grants him permission to investigate "any links and/or coordination between the Russian government and individuals associated with the campaign."

According to the order, "any matters that arose or may arise directly from the investigation" are also fair game.

Giuliani's questions about Mueller's authority stem from a prevailing suspicion between Trump and his close allies — including Giuliani himself — that the Russia probe is politically motivated and biased against the president.

Trump, who does not hide his disdain for the Democratic Party, has frequently accused Mueller of hiring Democrats who, in Trump's mind, have an ax to grind against him.

The president has previously expressed his willingness to speak to Mueller, but frequently oscillates from one position to another on that issue. And since Giuliani was brought on board, the president's stance has only become less clear.

If Trump ultimately decides not to voluntarily cooperate with the Mueller probe, the special counsel could subpoena the president. Giuliani says Trump would try to fight the subpoena, and if that happens, the matter could eventually reach the Supreme Court. Incidentally, Trump is poised to announce his second pick for the high court on Monday.