President Donald Trump's personal lawyer Rudy Giuliani is at the center of an explosive whistleblower complaint alleging that Trump repeatedly pressured Ukrainian President Volodymyr Zelensky to investigate former Vice President Joe Biden and his son ahead of the 2020 election.

Trump has said his conversation was innocent and related strictly to government business and the US's interest in rooting out corruption in Ukraine.

But Giuliani's involvement in the matter raises questions: "Why would a private citizen — not an employee of the United States — be involved in this unless he was working for the personal political benefit of his client, Donald Trump?" one Justice Department veteran told Insider.

Giuliani's status as Trump's personal fixer makes it difficult for those outside Trump's inner circle to gauge the nature of his work for Trump. But there's no doubt within the administration about the extent of his involvement.

"This was his show," one White House official told Insider on Friday. "He's been the president's top deputy on this since the get-go."

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He's a prolific TV presence, he relentlessly goes to bat for President Donald Trump, he swats down Trump's enemies, and he's the fixer Trump dispatched on a personal mission that has ignited a political firestorm.

Rudy Giuliani is the new Michael Cohen.

Giuliani is emerging as the key to understanding the brewing scandal over Trump's alleged efforts to get a foreign government to interfere in an upcoming presidential election.

Giuliani, like the now incarcerated Cohen, is Trump's personal lawyer and fixer. He and Trump are at the center of an explosive whistleblower complaint alleging that Trump asked Ukrainian President Volodymyr Zelensky to investigate former Vice President Joe Biden ahead of the 2020 election.

The intelligence official who filed the complaint said they were "deeply concerned" Trump's actions constituted "a serious or flagrant problem, abuse, or violation of law or Executive Order" that "does not include differences of opinions concerning public policy matters," which is consistent with the definition of an "urgent concern" under federal law.

Giuliani has said publicly on multiple occasions that he's pushed Ukrainian officials to investigate Biden and turn over dirt on him to the Trump team. If that's the case, legal experts said, the former New York mayor could be in legal jeopardy for potentially violating federal campaign-finance laws.

Read more: 'Lawyer up': DOJ veterans have one piece of advice for Trump and Giuliani amid the Ukraine whistleblower scandal

Giuliani's involvement in the Ukraine scandal also doesn't bode well for Trump because it undercuts the central premise of the president's defense: His conversation with Zelensky related strictly to government business and Trump's interest in rooting out corruption in Ukraine.

The complaint paints an intricate portrait of "the fact that Giuliani was driving a lot of this and doing the legwork for Trump" in Ukraine, Elie Honig, a former federal prosecutor from the Southern District of New York, told Insider. "But why would a private citizen — not an employee of the United States — be involved in this unless he was working for the personal political benefit of his client, Donald Trump?"

Giuliani's status as Trump's personal fixer makes it difficult for those outside Trump's inner circle to gauge the nature of his work for the president.

"My only knowledge of what Mr. Giuliani does — I have to be honest with you — I get from the TV or the news media," Joseph Maguire, the acting director of national intelligence, said as he testified on the complaint before the House Intelligence Committee on Thursday. "I'm not aware of what he does for the president."

But while the former New York mayor's role in the matter is puzzling, there's no doubt within the administration about the extent of his involvement.

"This was his show," one White House official told Insider on Friday. "He's been the president's top deputy on this since the get-go."

The whistleblower's complaint mentions Giuliani 31 times and describes him as a "central figure" in Trump's effort. It said Attorney General William Barr "appears to be involved as well."

The complaint said Trump referred "explicitly" to Giuliani and Barr as "his personal envoys on these matters" while speaking with Zelensky.

Read more: 2 key facts refute Trump's conspiracy theories about the Ukraine scandal

The document went on to outline the myriad meetings and contacts that it said Giuliani had with both US and Ukraine government officials to push Trump's personal agenda.

He met with Zelensky's aide Andriy Yermak in Madrid in August, according to the complaint, as part of a "direct follow-up" to Trump's call with Zelensky about the "cases" they had discussed.

Giuliani reached out to several other Zelensky advisers, but it's unclear whether they ultimately met or what was discussed.

Multiple US officials told the whistleblower that at least two of Zelensky's aides planned to travel to Washington, DC, in mid-August.

Giuliani spoke with Yuriy Lutsenko, Ukraine's prosecutor general at the time, at least twice this year.

In 2018, Giuliani spoke with former Ukrainian Prosecutor General Viktor Shokin, who is at the center of Trump's and Giuliani's unsubstantiated allegations of corruption against Biden.

He planned to travel to Ukraine in May to press the Ukrainian government to investigate Biden but called off the trip at the last minute after a public firestorm.

US officials told the whistleblower they were deeply concerned by Giuliani dealing with Ukraine, viewing it as a circumvention of normal national-security processes.

They said State Department officials spoke with Giuliani to try to "contain the damage" to national security.

Multiple US officials told the whistleblower Ukraine was led to believe that a call or meeting between the two presidents "would depend on whether Zelensky showed willingness to 'play ball' on the issues that had been publicly aired by Mr. Lutsenko and Mr. Giuliani."

The complaint also discusses Giuliani's public criticism of Marie Yovanovitch, a career diplomat who served as US ambassador to Ukraine from 2016 until she was recalled by the State Department in May.

Read more: The US's top intelligence watchdog found Trump's conduct so alarming it could expose him to blackmail

Giuliani has fiercely defended himself in the wake of the complaint's release. In an interview with CNN on Thursday, he referred to the complaint as "crap" and rejected its charges against him. And as the firestorm around his role in the scandal built, Giuliani began pointing a finger at the State Department and saying it was responsible for urging him to meet with Zelensky's aide in Madrid.

"The whistleblower falsely alleges that I was operating on my own," Giuliani told Fox News host Laura Ingraham on Thursday night. "Well, I wasn't operating on my own."

He went on to read aloud from what appear to be text messages with Kurt Volker, the US special representative to Ukraine, as evidence he was asked to meet with Yermak. He also disputed the notion that the meeting was a "follow-up" to Trump's July 25 call with Zelensky.

Read more: Trump is facing the biggest firestorm of his presidency because his own White House staffers blew the whistle on him

Giuliani told Ingraham the State Department "basically knew everything I was doing" and suggested he should get "some kind of an award" for his efforts.

A State Department spokesperson told The Wall Street Journal Volker put Giuliani in touch with Yermak at Yermak's request. "Mr. Giuliani is a private citizen and acts in a personal capacity as a lawyer for President Trump," the spokesman said. "He does not speak on behalf of the U.S. Government."

According to The Journal, after Yermak got Giuliani's number from the State Department, he called Giuliani and asked if, during TV appearances, he could tone down criticism of corruption in Ukraine because it was putting pressure on Zelensky's government.

Giuliani responded by suggesting Ukraine investigate Hunter Biden's ties to the Ukrainian natural-gas company, Burisma, according to The Journal.

The State Department and Giuliani did not immediately respond to requests for comment.