Giuliani, former governor of neighboring New York and a past contender for the Republican presidential nomination, said Christie faced a moment of truth that could at the very least scuttle his own ambitions for the White House.
"The man has put his political career completely at risk if it turns out there is some evidence that he knew about it," Giuliani said on ABC's "This Week." "He had taken the complete risk that his political career is over."
Giuliani said he tended to believe Christie's contention he was unaware members of his staff might have arranged to have lanes closed on the George Washington Bridge in September as political payback to the mayor of Fort Lee, where monumental traffic jams occurred for nearly a week.
"I think it's pretty darn credible," Giuliani said. "He wouldn't make this blanket denial unless it's not true."
Christie's knowledge of the lane closures will be the topic of hearings in the New Jersey state Legislature this week, which could lead to ouster from office.
Assemblyman John Wisniewski, who is leading the state investigation into the closure, said Christie could face consequences if he is found to have indeed used the closure as retribution against the mayor of Fort Lee for his refusal to endorse Christie's re-election.
"Using the George Washington Bridge, a public resource, to exact a political vendetta, is a crime," Wisniewski told NBC News. "Having people use their official position to have a political game is a crime. So if those tie back to the governor in any way, it clearly becomes an impeachable offense."
The Newark (N.J.) Star-Ledger said Wisniewski's Transportation Committee is set to have its subpoena power renewed this week and continue the probe that last week featured former Christie administration official David Wildstein invoking his Fifth Amendment right to refuse to answer potentially incriminating questions.
"He [Christie] knew there was an investigation," Wisniewski told NBC. "He knew people were looking at it, and his senior staff was involved. He expects us to believe he knew nothing? I just find that implausible."
Christie's travails have also been having a disruptive effect on the early stages of the 2016 presidential race. Christie was one of the early favorites for the Republican nomination; however GOP operatives said the idea of him using his power to cudgel an entire city would turn off a lot of voters, the report said.
"The Republicans would like to replace [President Obama] with somebody who shuts down the busiest bridge on planet Earth -- endangering how many lives? -- simply to grind a political ax," Steve Deace, a conservative activist in Iowa told NBC.
NBC said Christie has long had skeptics among the harder right wing for his stance on social issues, and the bridge flap has provided an opportunity for operatives for other prospective candidates to take fresh swipes at him.