President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE has turned to former New York City Mayor Rudy Giuliani for help with the Russia investigation, which increasingly poses danger for the president and his inner circle.

Here’s how Giuliani’s addition to Trump’s legal team could affect the president’s approach to the probe.

Giuliani has a history with Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE

One of Giuliani’s top tasks is to leverage his long-standing ties to special counsel Robert Mueller to smooth the frayed relationship with his team.

The Sept. 11, 2001, terrorist attacks were a turning point in the careers of Giuliani, who was dubbed “America’s Mayor” for his response, and in the career Mueller, who built his reputation as a trusted lawman as FBI director.

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Before he was mayor, Giuliani served as U.S. attorney for the Southern District of New York — the same office that recently raided Trump’s personal attorney, Michael Cohen.

The raid on Cohen piqued the president’s anger and damaged the relationship between his lawyers and Mueller’s team.

In Trump’s eyes, Giuliani has the stature to get things back on track.

“He can keep Trump comfortable, which is more than can be said for almost any other lawyer he might have hired,” said Walter Olson, a legal scholar at the Cato Institute.

But others doubt that Giuliani will have sway with Mueller.

Former FBI Director James Comey James Brien ComeyDemocrats fear Russia interference could spoil bid to retake Senate Book: FBI sex crimes investigator helped trigger October 2016 public probe of Clinton emails Trump jabs at FBI director over testimony on Russia, antifa MORE, who worked under Giuliani in the Manhattan prosecutor’s office, criticized his former boss for leaving "a trail of resentment” among federal judges in New York.

“Though Giuliani’s confidence was exciting, it fed an imperial style that severely narrowed the circle of people with whom he interacted, something I didn’t realize was dangerous until much later: a leader needs the truth, but an emperor does not consistently hear it from his underlings,” he wrote in his new book “A Higher Loyalty.”

Trump is turning to people he knows

Giuliani’s hiring is part of a trend of Trump filling his team with personal confidants and loyalists.

Trump recently named John Bolton and Larry Kudlow, both prominent conservative voices on cable news, as his national security adviser and top economic aide — replacing two men with whom did not have a prior relationship.

The president also did not know his previous lead lawyer in the Russia probe, John Dowd, who quit in large part because Trump did not heed his advice.

Giuliani has long been a close ally of Trump, advising him informally on a wide range of matters, including his first travel ban against several majority-Muslim nations. He even vied to be Trump’s original secretary of State, but was passed over for the job.

“The only thing that is surprising about this to anyone in the president’s orbit is that it took this long for Rudy to join Trump’s team in an official capacity,” said a former administration official.

People in Trump World believe Giuliani will be able to tell Trump “no” while also remaining in his good graces because of their personal relationship.

But some fear Giuliani has been out of the game too long — he has not practiced law full-time in decades — to strike fear into Mueller’s team of seasoned attorneys.

Giuliani is an attack dog

There’s another side to Giuliani that Trump likes — he is not afraid of a fight.

The 73-year-old was one of Trump’s most vocal surrogates during the 2016 presidential race, tearing into Democratic nominee Hillary Clinton Hillary Diane Rodham ClintonButtigieg stands in as Pence for Harris's debate practice Senate GOP sees early Supreme Court vote as political booster shot Poll: 51 percent of voters want to abolish the electoral college MORE at campaign rally after campaign rally.

He made a string of controversial statements, including a false claim that Clinton did not go to the World Trade Center site after the Sept. 11 attacks. Those comments drew criticism but endeared him to Trump.

Expect Giuliani to appear on cable news shows to defend the president, and possibly attack the probe into Russia’s interference in the 2016 election.

One of his past statements could be a problem

Now that he has officially joined Trump’s legal team, one of Giuliani’s campaign-trail jabs may come back to haunt him.

The ex-mayor hinted during an Oct. 26, 2016, Fox News interview that he has insider knowledge about the federal probe into Clinton’s use of a personal email server as secretary of State.

“I’m talking about some pretty big surprises,” he said.

Then-FBI Director Comey announced two days later the bureau had resumed its investigation into Clinton’s emails.

Giuliani’s comments sparked speculation that he had been tipped off by the FBI, something he denied.

But the comments worried top brass at the FBI, which launched an investigation at Comey’s direction into whether Giuliani’s comments were the result of a leak.

“I got fired before it was finished, but I know that I asked that it be investigated,” Comey told MSNBC’s Rachel Maddow this week.

If Giuliani remains under suspicion, that could complicate his interactions with federal authorities as Trump’s lawyer.

Attorney General Giuliani? Not so fast

Giuliani’s addition to the legal team had some Trump allies buzzing about the possibility that he could replace Jeff Sessions Jefferson (Jeff) Beauregard SessionsGOP set to release controversial Biden report Trump's policies on refugees are as simple as ABCs Ocasio-Cortez, Velázquez call for convention to decide Puerto Rico status MORE as attorney general.

Trump during the transition reportedly offered the job to Giuliani, who turned it down in hopes of being named secretary of State.

Sessions has never escaped Trump’s doghouse over his decision to recuse himself from the Russia probe and the president reportedly mused last summer about giving the job to Giuliani.

But such a move is doubtful, in large part because the former mayor could struggle to win a confirmation vote in the Senate.

Giuliani would face tough scrutiny over the paid speeches he gave while in the private sector and his views on abortion have never gelled with socially conservative Republicans.