Washington (CNN) It was two weeks after the release of the Mueller report, and Rudy Giuliani was eager to chat. As President Donald Trump's personal lawyer, Giuliani has spent much of the past year going on TV to defend his client on a number of fronts— casting doubt on the special counsel's investigation, taking shots at Trump's former fixer Michael Cohen, and generally serving as the President's chief public defender.

Now, at 8 pm on a Thursday night, on the phone from New York, Giuliani wanted to talk about something else entirely: Joe Biden.

The former vice president and current frontrunner for the Democratic presidential nomination could pose the biggest threat to President Donald Trump's re-election effort. And Giuliani was looking to take some shots— not for any policies Biden has or even the recent controversy over Biden's physical interactions with women.

"I was shocked how much attention that stuff got," Giuliani said dismissively after broaching the topic. "He didn't have sex with them. He didn't ask anyone to have sex with him."

Instead, Giuliani was more interested in reviving an attack he's leveled against Biden's intelligence for more than a decade: "I think a lot of his gaffes come from the fact that he's not as smart as a lot of the people he's dealing with."

Giuliani paused before adding: "But that's not a sin."

After a year of playing defense on the Mueller probe, Giuliani has reason to feel he's coming off a big win. He, like the President, always saw the Trump-Russia case as more of a political problem than a legal one, and that muddying the waters would be enough to see his client through. In a way, he's been proven right. According to a CNN poll conducted in the days after attorney general William Barr released his summary findings of the Mueller report, among people who have heard or read "a great deal" about the report, 56% said Trump and his campaign have been exonerated on collusion.

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Now, as Trump and his team begin switching from defense to offense, with an eye to the 2020 campaign, Giuliani is also shifting tactics, and taking an active role in prosecuting a public case against Trump's political opponents. The outspoken former New York mayor operates from a unique position as the President's personal lawyer, set apart from both the White House and official campaign.

That gives Giuliani -- with decades of experience as a federal prosecutor and a lengthy list of media contacts -- the chance to play a free-agent attack dog on Trump's behalf.

Among his top priorities is uncovering what he believes are the corrupt origins of the FBI investigation into the Trump campaign. That's led him to seek out a connection to Ukraine, where he claims without proof that government officials worked with Democrats to spread misinformation about Trump.

Giuliani tells CNN that a "well-regarded investigator" he's known for years clued him into the connection, and put him in touch with current and former Ukrainian officials, who he says he's interviewed over Skype and in person. (In 2017, both the Democratic National Committee and a contractor denied working with Ukrainians .)

While searching for evidence, Giuliani says he also stumbled on what he claims is a damaging story about Biden.

Biden in Ukraine

What Giuliani began piecing together was that in 2016, while then-vice president Biden was pressuring the Ukrainian government to oust its top prosecutor as part of a broad anti-corruption push by the US, his son Hunter Biden was on the board of a Ukrainian company under investigation by that same prosecutor.

Within a year of the prosecutor's removal, Ukraine's new prosecutor general dismissed the case against the company, Burisma Holdings, a natural gas company controlled by one of Ukraine's top oligarchs.

While there's never been any evidence that Biden acted improperly, to Giuliani the story was a political gift too good to pass up, especially as Biden prepared to run for the White House.

"I said, 'Holy sh*t, what's that all about?'" he told CNN.

But Giuliani's story is littered with holes. According to a report from Bloomberg , the Ukrainian government's case against Burisma had been "dormant" since 2014, two years before Biden successfully pushed to remove the prosecutor general. Biden was also joined in his anti-corruption push against the prosecutor by numerous leaders in Europe as well as the International Monetary Fund -- none of whom had any family ties to Burisma.

That hasn't stopped Giuliani from publicly calling for the Department of Justice to start investigating Biden, something he says the President agrees with. "He has no particular interest in the Biden part of it," Giuliani said of Trump. "It's just something there that should be investigated."

The Justice Department declined to comment.

Giuliani has also been shopping the story to journalists around Washington. The story floated around right-wing circles for a few weeks until May 1, when the New York Times delved deeper into the details, reporting that State Department officials at the time were concerned "that the connection could complicate" Biden's diplomacy.

The Biden campaign declined to comment further and referred CNN to a statement a spokeswoman provided to the Times claiming Biden acted on Ukraine "without any regard for how it would or would not impact" Hunter's business interests. Attempts to reach Hunter Biden were unsuccessful.

A history of harsh words

Giuliani has a history of going after Biden. The two men, both in their mid-70s, have known each other for decades and worked together on what is perhaps the biggest political liability for Biden -- the 1994 crime bill he helped craft. As New York's crime-fighting mayor, Giuliani played a crucial role in getting bipartisan support for the bill, which among other things funded more police officers and added a number of new offenses to the federal registry, targeting gangs and repeat offenders.

Their interactions remained sparse and professional until they both decided to run for President in 2007. During a Democratic debate that fall, with Giuliani then leading the Republican presidential field, Biden quipped that there were just three things the former New York City mayor uses in a sentence: "a noun, a verb and 9/11."

The comment cut to the core of what ended up being one of Giuliani's major weaknesses as a national candidate— beyond his leadership after the terrorist attacks, what more did the former New York mayor have to offer?

Biden's remarks infuriated Giuliani, whose campaign fired back with an angry statement from its top spokeswoman, claiming that she had a better shot at becoming president than Biden. Within three months though, Giuliani's presidential campaign had imploded, and with it any political future for "America's Mayor." One former aide tells CNN that the experience left Giuliani more personally embittered toward Democrats, including Biden.

In the years since, Giuliani has consistently targeted Biden's intelligence, questioning the Vice President's "mental capacity" during the Obama administration and telling HuffPost last summer that Biden is a "moron" and a "mentally deficient idiot." He also told CNN's Chris Cuomo that Biden was "dumb."

Mike DuHaime, Giuliani's 2008 campaign manager, confirms his old boss's long-held view of Biden. "I do not know the genesis of the acrimony, but it goes back a long time," he said.

Back in the game

After 2008, Giuliani continued building his private management and security consulting business. He also remained a partner at the law firm Bracewell and Giuliani until 2016. His national profile diminished as he was relegated to pundit-status on cable news, where he'd often rail against the Obama administration.

Trump's rise however presented Giuliani an opportunity for one last shot at presidential influence. Though he was slow to endorse Trump in 2016, holding off until the New York primary in April, Giuliani's anti-Hillary Clinton speech at the Republican National Convention solidified his position in Trump's inner circle.

His selection as Trump's personal attorney in April 2018 drew plenty of criticism, particularly as Giuliani made his fair share of gaffes along the way— "Truth isn't truth," he told an incredulous Chuck Todd of NBC's Meet the Press shortly after taking the role.

But he's proven a valuable asset to Trump. People who are close to him describe Giuliani as a hybrid figure in Trump's kitchen cabinet: a political operator, a true-believer, a lawyer, a campaign surrogate, and a media personality. He speaks often with the President, and the two men share New York backgrounds and sensibilities, though a former aide to Giuliani says they were not close before Trump launched his bid for the White House.

Defending Trump in the court of public opinion on the President's legal issues was an obvious place for a loyal member of the team like Giuliani to go. As the head of Trump's legal team, he gave the public what the President wanted to hear, even when he came across as bombastic or not fully put together.

Despite the gaffes, Giuliani never strayed from the main message. He says he believed from the beginning Trump wasn't guilty. "I always knew personally, as best you can know anything, that the Russian conspiracy thing was bullsh*t. I was on the campaign for four or five months," Giuliani told CNN. "If it was going on, I would have known it."

With Giuliani's gift for gab and willingness to adopt an ever shifting defense of the behavior of Trump and his associates, his pivot to attacking Trump's 2020 foes is a logical next step. The Trump campaign did not respond to a request for comment on whether Giuliani will have an official role, and people close to him say he's more likely to be an unofficial surrogate -- where nobody but the President can tell him what to do. And that may be where both Trump and Giuliani want him.

"He's the guy everyone wants to come in at the end of a tough campaign," Tony Carbonetti, a longtime Giuliani adviser, told CNN. "He'll punch the other guy in the face."