The former U.S. attorney and mayor of New York maintained, however, that there was no legitimate reason to appoint special counsel Robert Mueller to led a probe of potential links between President Donald Trump's election campaign and Russia.
"It was really about the firing of Comey that led to [Deputy Attorney General Rod] Rosenstein's decision" to appoint the special counsel, Giuliani said Friday in a CNN interview.
Comey was fired on May 9, 2017. Two weeks later, Rosenstein appointed the special counsel to the FBI's ongoing probe of Russian election meddling.
Shortly after Comey's termination, the White House said Trump's action was based on a letter from Rosenstein criticizing Comey's handling of an investigation into former presidential candidate Hillary Clinton's emails.
But in an NBC Nightly News interview two days later, Trump gave a different explanation. "In fact, when I decided to just do it, I said to myself, I said, 'You know, this Russia thing with Trump and Russia is a made up story,'" Trump told NBC.
Giuliani told CNN the firing was an insufficient reason to appoint a special counsel. "No reason to investigate. The president has complete discretion to fire anybody he wants," he said.
While conceding that Trump could potentially obstruct justice — a facet of the investigation that has reportedly grown more prominent over time — Giuliani said Comey's firing, regardless of the president's reason for doing so, wouldn't count as such.
"In the case of firing a subordinate who's going to be replaced by somebody else on an acting position immediately, ... it doesn't matter if, in fact, it can't result in anything," Giuliani said.
Giuliani also said the president cannot be indicted, citing Justice Department precedent.
Earlier on CNN, Giuliani said he learned that the special counsel has whittled down the number of subjects about which Trump could be questioned in a potential interview.
Jay Sekulow, another member of Trump's legal team, had assembled information from the special counsel into a list of 49 potential questions for the president, according to The Washington Post. The questions, revealed by The New York Times, touched on Attorney General Jeff Sessions' recusal from the investigation, as well as the firings of Comey and former national security advisor Michael Flynn.
Giuliani said a number of the question categories have been removed from the list that could potentially be put to Trump.