President Donald Trump's personal defense attorney, Rudy Giuliani, took heavy aim at Michael Cohen's credibility on Sunday, calling him a "liar" and a "scoundrel."

Giuliani rejected Cohen's recent reported claim that Trump knew in advance of a meeting between top campaign officials and a Russian lawyer offering dirt on Hillary Clinton ahead of the 2016 election.

Giuliani called the claim "flat out untrue."

In May, Giuliani called Cohen an 'honest' and 'honorable' lawyer, but said his perception changed after it was revealed last week that Cohen secretly recorded a conversation with Trump about payments to a former Playboy model.

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Rudy Giuliani, President Donald Trump's personal defense attorney, set his sights Sunday on the credibility of Michael Cohen, Trump's former longtime lawyer.

Cohen is at the center of a federal criminal investigation into whether he committed bank fraud, wire fraud, and campaign finance violations while working for Trump during the election.

Cohen initially demonstrated an intense loyalty to Trump, once saying that he would "take a bullet" for the president. But he has soured on Trump and his allies in recent weeks, particularly after the president and the White House distanced themselves from Cohen after the FBI executed a raid on his properties on April and seized thousands of documents and records.

Most recently, CNN reported that Cohen claims Trump knew in advance of a June 2016 Trump Tower meeting between senior campaign officials and a Kremlin-connected lawyer offering dirt on then Democratic nominee Hillary Clinton.

Cohen is said to claim that he was one of several people in the room when Donald Trump Jr. informed his father of the offer. Cohen reportedly says Trump greenlit the meeting. Trump Jr. was later one of several top campaign officials, including Jared Kushner and Paul Manafort, who attended the meeting.

That meeting is now one of the central threads of the special counsel Robert Mueller's investigation into Russia's election interference, whether the Trump campaign colluded with Moscow, and whether Trump sought to obstruct justice in the investigation.

During an interview on "Fox News Sunday," Giuliani called Cohen a "scoundrel" and a "liar." He also dismissed Cohen's claims about Trump's knowledge of the June 2016 meeting as "flat out untrue."

Giuliani's appearance on Sunday came after he told ABC News Trump's legal team had split with Cohen and ended the joint defense agreement the two parties previously shared.

"He's destroyed himself as a witness. I've prosecuted 5,000 cases. I'd never prosecute a case on this guy's testimony," Giuliani said. "He's contradicted himself so many times, you'd begin the cross-examination by saying 'which lies are you going to tell us today, Michael?'"

He also called Giuliani a "pathological manipulator."

The former New York mayor's criticisms of Cohen come just a few months after he called him "an honest, honorable lawyer" in a separate ABC News interview.

When Fox News host Chris Wallace brought up the discrepancy between Giuliani's previous statements and his comments on Sunday, Giuliani said his perception of the former Trump lawyer had changed amid Cohen's recent actions.

He was referring to revelations last week that Cohen secretly recorded a conversation with Trump about payments to Karen McDougal, a former Playboy model who claims to have had an affair with Trump years ago.

The conversation happened shortly before the 2016 election, and Trump and Cohen can be heard on the tape discussing ways to pay McDougal. She was ultimately paid $150,000 by the publisher of a Trump-friendly tabloid, but her story was never published, a technique known as "catch-and-kill."

"Here's what happened," Giuliani said Sunday. "I found out, as everyone did, that he was surreptitiously recording his clients, which is a disbarrable offense. Obviously, if I knew that I never would have said he was a reputable lawyer, I'd have said he was a scoundrel."

Legal experts told Business Insider last week that they found it bizarre that Cohen felt the need to record a conversation with Trump, and some said they had never heard of an attorney secretly recording a client.

The New York State Bar Association calls such secret recording unethical. New York, where the taping reportedly took place, legally requires only one person's consent for such a recording. So Cohen's actions were not illegal.

Giuliani said the president is "disappointed" over Cohen's secret recordings, but said that they clear him from any suspicion of wrongdoing. Trump tweeted Friday Cohen was just "trying to make up stories" to distract from his own troubles.

He also suggested last week that the recording vindicates him.

"Inconceivable that the government would break into a lawyer's office (early in the morning) - almost unheard of," Trump tweeted. "Even more inconceivable that a lawyer would tape a client - totally unheard of & perhaps illegal. The good news is that your favorite President did nothing wrong!"

Cohen has recently made a concerted effort to distance himself from Trump.

In addition to publicly coming out against Trump's actions, like the "zero tolerance" immigration policy, Cohen recently told ABC News George Stephanopoulos that his family "and this country" have his "first loyalty."

Watch the full interview below: