Former New York City Mayor Rudy Giuliani talks with reporters in the lobby of Trump Tower in New York last year. He has joined President Trump's legal defense team.

NEW YORK — Rudy Giuliani, once known as “America’s Mayor” and hailed for helping unite a wounded city after Sept. 11, has become the in-your-face face of President Donald Trump’s aggressive new legal team.

Giuliani, who is bonded with the president by a particular brand of New York bravado, has escalated Trump’s attacks on the Department of Justice, pushed for strict limits on special counsel Robert Mueller’s Russia probe and upended White House legal strategy. Giuliani and Trump cut out senior West Wing aides this week as they hashed out plans to combat what they see as an existential threat to the presidency.

Giuliani’s bold offensive — on display in a series of cable news appearances in which he unleashed broadsides on the very law enforcement officers with whom he once worked — underscored the thoroughness of his transformation from moderate Republican mayor of a liberal city to fiery conservative hero.

“Russian collusion is total fake news,” Giuliani, a former U.S. attorney, told Fox News. “Unfortunately, it has become the basis of the investigation. And Mueller owes us a report saying that Russia collusion means nothing, it didn’t happen. That means the whole investigation was totally unnecessary.”

Giuliani has quickly become the dominant figure on the president’s reshuffled legal team as Trump stocks his political inner circle with familiar, TV-ready faces. The two have had several private conversations in recent days in which Giuliani fanned Trump’s anger with Mueller’s probe, according to two people familiar with their conversations who spoke on condition of anonymity because they weren’t authorized to publicly discuss them. Giuliani has warned Trump against sitting down for an interview with Mueller and has suggested that, at a minimum, the president place limits on his level of cooperation.

Giuliani has warned Trump that he fears that the president’s longtime personal attorney, Michael Cohen, may flip on him. He has urged Trump to cut off communications with Cohen, according to a person close to Giuliani but not authorized to discuss the talks publicly. After an FBI raid on Cohen’s office and home, Giuliani also indicated that he wanted to change the discussion surrounding the $130,000 payment that Cohen made to porn actress Stormy Daniels to buy her silence about a sexual tryst with Trump. Giuliani did so with a jaw-dropping interview with Sean Hannity on Wednesday.

Giuliani’s remarks — that Trump knew about the payment and had repaid Cohen for it — seemed to contradict Trump’s past statements. But he argued that it removed legal peril over a possible campaign finance violation, a claim some legal experts have questioned. Trump was pleased with Giuliani’s performance, according to a person familiar with his views but not authorized to speak publicly about private conversations.