Last week, President Trump tapped longtime friend and former New York City Mayor Rudy Giuliani to join his legal team in hopes of negotiating an end to Robert Mueller’s probe into 2016 Russian election meddling.

“I’m doing it because I hope we can negotiate an end to this for the good of the country and because I have high regard for the president and for Bob Mueller,” Giuliani told the Washington Post.

There’s just one problem: Giuliani would be a great witness for investigators.

Giuliani was one of Trump’s loudest supporters on the campaign trail; he was early to criticize the Justice Department and the FBI for their handling of the Hillary Clinton email server investigation, and even claimed to be in touch with FBI agents who he said shared in his frustration. He attended rallies and made regular appearances on radio and cable shows on Trump’s behalf.

To avoid discussing his time on the campaign with special counsel Mueller, he may try to hide behind his new attorney-client privilege.

“It’s hard to contemplate a way in which Giuliani would not have evidence that is relevant to Mr. Mueller’s investigation”

“The investigation has so many moving parts that it’s hard to contemplate a way in which Giuliani would not have evidence that is relevant to Mr. Mueller’s investigation,” said Washington, D.C.-based attorney Josh Rosenstein, an expert in foreign lobbying regulation.

Giuliani began to advise Trump in early 2016 about what language to use at campaign events, and officially endorsed Trump for president in April, soon after dropping his own campaign for president. He then became a very vocal surrogate churning out “deep state” messaging and lock-her-up chants during the final months before the November election.

As a former federal prosecutor, Giuliani is well connected in the law enforcement community. He had prior knowledge of a “surprise” event that would shake up the election before former FBI Director James Comey announced that he was reopening the agency’s probe of the Clinton email server, which may be of interest to both Muller and House Republicans.

“We’ve got a couple of things up our sleeve that should turn this thing around,” Giuliani said on Oct. 26, 2016. Two days later, Comey sent a letter to Congress announcing Clinton was back under investigation, less than two weeks before the election.

Giuliani said the Justice Department decision not to prosecute Clinton for the email server was “worse than Watergate.”

“You have outraged FBI agents that talk to me,” Giuliani said on Fox News on Nov. 2. “They are outraged at being turned down by the Justice Department to open a grand jury. They are convinced that Loretta Lynch has corrupted the Justice Department. You've got people in the Justice Department in charge of this investigation who are defense lawyers for Clinton people.”

This may all be of interest to Mueller because when Trump fired Comey in May 2017, the official reason given was Comey’s handling of the Clinton email controversy, the details of which seemed to have been leaked to Giuliani before they were made public. (Trump later told NBC’s Lester Holt that he was thinking about the Russia investigation when he fired Comey.) The special counsel’s office is looking into whether Trump obstructed justice when he let Comey go as a way to try to derail the investigation.

Legal experts say if Giuliani is asked to testify, he will try to use attorney-client privilege to avoid divulging information related to his time on the campaign trail.

“He’s going to say, ‘we’ve been discussing that as part of the defense and the facts are now interwoven into the defense strategy, so I have attorney-client privilege about what’s happened',” said Ron Oleynik, the head of Holland & Knight’s international trade practice, based in D.C. “That’s for the judge to decide.”

Attorney-client privilege has been at the center of the debate in federal court in New York City about who gets to review the documents seized from Cohen earlier this month. The judge in the case is allowing prosecutors and Cohen’s lawyers to move forward with their review of the material simultaneously.

It’s not just Mueller who may be interested in Giuliani’s testimony. House Republicans sent a letter to the Justice Department last week urging them to prosecute Comey for what they say was a failure to bring charges against Clinton for the misuse of her email server, a subject Giuliani has intimate knowledge of.

“What does that mean for his ability to represent the president?” Rosenstein said. “That's a much more complicated question. Generally attorneys representing clients will try to avoid becoming fact witnesses to events that are of issue in an investigation. Once you become a fact witness, then it’s very possible even as the attorney you might get dragged into the middle of the fight.”

The special counsel’s office would not confirm whether Giuliani has been interviewed yet.