Former New York Mayor Rudy Giuliani is increasingly emerging as the weakest link in the ongoing scandal over President Donald Trump's dealings with Ukraine for three main reasons.

Giuliani has been implicated by several government officials as Trump's main envoy in an effort to solicit Ukraine's interference in the 2020 election.

He is also under criminal investigation by the US attorney's office in the Southern District of New York over his role in the ouster of a top US diplomat in Ukraine.

And it recently emerged that the investigation includes a counterintelligence probe, a stunning revelation that could change the entire nature of the inquiry.

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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.

And like the now-incarcerated Cohen, it recently surfaced that Giuliani is under criminal investigation by the US attorney's office in the Southern District of New York.

The news is just the latest development indicating that Giuliani is the biggest weak link in the brewing Trump-Ukraine saga.

At the heart of the controversy are Trump's and Giuliani's efforts to leverage US foreign policy to pressure Ukraine to launch investigations that would politically benefit the president.

Read more: These are the key players you need to know to make sense of the Trump impeachment inquiry

In particular, the president wants the Ukrainian government to investigate Burisma Holdings — a Ukrainian natural gas company associated with former Vice President Joe Biden's son — and an unfounded conspiracy theory that the Democratic Party colluded with Ukraine to interfere in the 2016 election.

Trump raised the issue during a July 25 phone call with Ukrainian President Volodymyr Zelensky. The president ordered his administration to hold a nearly $400 million military-aid package to Ukraine days before the call.

During the conversation, Trump reminded Zelensky the US "does a lot for Ukraine" and followed up by asking him to "do us a favor, though," and investigate the Bidens and the origins of the Russia probe.

The details of the phone call, outlined in an unprecedented whistleblower complaint against Trump, sparked public outrage and allegations that the president and his lawyer were soliciting foreign interference in the upcoming election.

But the phone call was just one thread in Trump's and Giuliani's plan. They had put the wheels in motion months before.

Former U.S. ambassador to Ukraine Marie Yovanovitch, arrives on Capitol Hill, Friday, Oct. 11, 2019, in Washington, as she is scheduled to testify before congressional lawmakers on Friday as part of the House impeachment inquiry into President Donald Trump Scott J. Applewhite/AP

Giuliani v. Yovanovitch

In the spring, Giuliani gave Secretary of State Mike Pompeo a folder, dated March 28, of unfounded conspiracies against the Bidens, as well as allegations of impropriety against Marie Yovanovitch, then the US's ambassador to Ukraine.

In many ways, Yovanovitch is the central thread in the Giuliani-Ukraine scandal. She was abruptly recalled from her position in May after a tumultuous stint that included frequent clashes with Giuliani.

Their main disagreement stemmed from her refusal to facilitate the former New York mayor's efforts to use official channels to push the Ukrainian government to dig up damaging information on the Bidens. Yovanovitch was also opposed to Yuriy Lutsenko, then the Ukrainian prosecutor general who was in cahoots with Giuliani.

"She refused to allow her embassy to be dragged into some sort of effort to concoct dirt for political purposes," a former official told The Guardian.

Yovanovitch testified to Congress that she was removed based on "unfounded and false claims by people with clearly questionable motives" and that the campaign against her was spearheaded by Trump and Giuliani beginning last spring.

Read more: 8 Trump officials made stunning revelations about how the president and Giuliani weaponized the State Department

Now, Yovanovitch's ouster is the focus of the SDNY's investigation into Giuliani. Specifically, prosecutors are looking at whether Giuliani was working on Lutsenko's behalf while trying to get Yovanovitch removed; if he was, it may have violated foreign lobbying laws.

Prosecutors are also interested in Giuliani's business interests in Ukraine. Giuliani's entanglement in the SDNY's probe first came out when two of his Ukrainian associates, the businessmen Lev Parnas and Igor Fruman, were arrested and charged last week with campaign-finance violations.

The two men are accused of trying to funnel foreign money into Republican political campaigns, and Parnas in particular also worked as Giuliani's fixer in Ukraine, where he focused on looking for dirt on Biden and Yovanovitch.

Parnas and Fruman's efforts on Giuliani's behalf also had a domestic thread. This week, the former GOP congressman Pete Sessions was subpoenaed in the SDNY investigation to turn over documents and other information about his interactions with Giuliani.

It's unclear what exactly Sessions' role is in the Ukraine saga, but the indictment against Parnas and Fruman could offer some clues. The charging document said the two men dangled an illegal campaign donation to a US congressman while looking for their "assistance in causing the US government to remove or recall the then-US ambassador to Ukraine."

In May 2018, around the time when Parnas and Fruman promised to raise $20,000 for the unidentified congressman, Sessions wrote a letter to Pompeo criticizing Yovanovitch and parroting right-wing talking points that she was working against Trump. A year later, Yovanovitch was recalled from Ukraine following what she characterized as a smear campaign by Giuliani, Trump, and others.

Read more: Mick Mulvaney publicly confirms Trump held up Ukraine aid for political gain

Multiple officials implicate Giuliani for running a shadow foreign-policy campaign

Giuliani's outsize role in the Ukraine controversy has puzzled ethics experts and government officials, given that he is the president's personal lawyer and a private citizen.

Those concerns doubled after the release of explosive text messages between three US diplomats — the former Special Representative to Ukraine Kurt Volker, the US's ambassador to the EU Gordon Sondland, and the US's chargé d'affaires in the Ukrainian embassy Bill Taylor — that showed just how intricately involved Giuliani was in running a shadow foreign policy campaign to help Trump's political ambitions.

It didn't hurt that Volker and Sondland were willing participants in Giuliani's efforts.

Read more: Gordon Sondland, a central figure in the Ukraine scandal, threw Trump and Giuliani under the bus in his opening statement to Congress

Volker was instrumental in setting up a channel of communication between Giuliani and Andriy Yermak, a key aide to Zelensky. And he and Sondland were both enthusiastic about helping Giuliani convey to the Ukrainian president that a good relationship with Trump was predicated on Ukraine pursuing politically motivated investigations against Trump's opponents.

When Taylor, the skeptic of the group and a career foreign-service officer, raised concerns about the US withholding military aid and conditioning a White House meeting on Zelensky caving to Trump's demands, he was shut down by Sondland.

Sondland and Volker have testified as part of Congress' impeachment inquiry into Trump, and both implicated Giuliani as being the president's main envoy with respect to Ukraine while distancing themselves from the controversy. Taylor is set to appear before lawmakers on Tuesday.

At least half a dozen other officials have also testified about the extent of Giuliani's involvement in Trump's pressure campaign on Ukraine.

Read more: Trump's presidency is disintegrating as he faces his worst 30 days since taking office

Former Mayor of New York Rudolph Giuliani during the Conference In Support Of Freedom and Democracy In Iran on June 30, 2018 in Paris, France. The speakers declared their support for the Iranian peoples uprising and the democratic alternative, the National Council of Resistance of Iran and called on the international community to adopt a firm policy against the mullahs regime and stand by the arisen people of Iran. (Photo by Anthony Devlin/Getty Images)

FBI sees Giuliani as a possible national security threat

Meanwhile, it recently surfaced that the criminal investigation into Giuliani also includes a counterintelligence component, a stunning revelation that could change the whole nature of the probe.

CNN reported on Wednesday that FBI investigators approached Kevin McCallion, an attorney in New York, earlier this year to ask about Giuliani's link to Parnas and Fruman and his business dealings in Ukraine.

Jeffrey Cramer, a longtime former federal prosecutor who spent 12 years at the Justice Department, told Insider the existence of a counterintelligence probe means investigators are "casting a wide net."

"Cases like these, and they aren't common, usually involve multiple schemes and numerous players," Cramer said. "As such, they touch upon many different possible criminal violations."

Read more: Rudy Giuliani urged Trump to turn over an exiled Muslim cleric to Erdogan, raising concerns that he was lobbying for Turkey

Asha Rangappa, a former FBI special agent, wrote on Twitter that the existence of a counterintelligence investigation into Giuliani indicates that the FBI believes "he may pose a national security threat to the United States."

Such investigations aren't rooted in suspicions that someone violated US laws, but rather in concerns that the individual was in contact with people linked to foreign intelligence, or that they're working to further the interests of a foreign power in the US, Rangappa added.

The bar for opening this type of investigation into Giuliani, who is a US national and the president's personal lawyer, is also higher than it is for probing foreign individuals.

"So basically, the FBI thinks something bad — and likely not 'unwitting' — is up," Rangappa wrote. "That Giuliani is a conduit for pushing the agendas of foreign intelligence and/or foreign interests."