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Former FBI director James Comey's explosive Senate testimony is more than enough to launch a case against U.S. President Donald Trump for obstruction of justice, says the man who served as counsel to Watergate prosecutors.

"The evidence provided by director Comey, in my view, is sufficient to establish the elements of the crime," Philip Allen Lacovara, a former deputy solicitor general in the U.S. Justice Department, told As It Happens host Carol Off.

Lacovara served as counsel to special prosecutors Archibald Cox and Leon Jaworski during the Watergate investigation into former U.S. president Richard Nixon.

Speaking before the Senate intelligence committee on Thursday, Comey dove into the heart of the fraught political controversy around his firing and whether Trump interfered in the bureau's Russia investigation, as he elaborated on written testimony delivered Wednesday.

Philip Allen Lacovara served as counsel to Watergate special prosecutors. (United Nations)

In that written testimony — which was based on detailed notes he kept of his interactions with the president — Comey describes several private meetings and conversations in which Trump allegedly demanded Comey's loyalty, and asked him to quash an agency probe into his national security adviser, Michael Flynn, who was forced to resign in February.

Order or request?

In one instance, Comey wrote that Trump asked his advisers to leave the Oval Office so that he and Comey could be alone, then said: "I hope you can see your way clear to letting this go, to letting Flynn go. He is a good guy. I hope you can let this go."

Under questioning from senators on Thursday, Comey said he interpreted that as an order from the president.

"I mean, this is a president of the United States with me alone saying, 'I hope this.' I took it as, this is what he wants me to do. I didn't obey that, but that's the way I took it," Comey said.

Lacovara agrees with Comey's interpretation of the situation, but says it ultimately doesn't matter.

"All the statute forbids is an endeavour to obstruct. So it isn't a question of whether he actually ordered him to do it, but whether he was trying, even by a strong suggestion from superior to subordinate, that this investigation be made to go away," he said.

Spectators and patrons wait in line to enter Shaw's Tavern in northwest Washington, D.C., as they watch as former James Comey testify. (Jim Watson/AFP/Getty Images)

It also doesn't matter that the FBI investigation continued, he said.

"It is unnecessary to show — and irrelevant, in fact — whether the endeavouring to obstruct an investigation was successful or not."

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What does matter, he said, is finding evidence Trump intended to halt the FBI probe, which he says "the president himself has supplied."

After firing Comey last month, Trump told CNN's Lester Holt he was thinking of "this Russia thing" when he made the decision.

"The White House had originally said that [Comey] was fired as a result of dissatisfaction with the way director Comey had handled the Hillary Clinton matter last July, and the president promptly undercut that defence as a sham," Lacovara said.

Who is more credible?

After the testimony wrapped up Thursday, Trump's lawyer Marc Kasowitz issued a written statement denying the allegations Trump asked Comey for loyalty and pushed him to let the investigation into Flynn go.

"The President never, in form or substance, directed or suggested that Mr. Comey stop investigating anyone," Kasowitz wrote.

Now it comes down to a matter of whose word can be trusted, Lacovara said.

"I think that public opinion polls show that there is a very substantial majority of the populace, including people who voted for him, who believe that President Trump does not really have much adherence to the truth and if he says something that's true, it's almost an accident," he said. "By contrast, director Comey is widely respected as an honest and honourable man."

Comey: 'I was honestly concerned that he might lie' 0:46

The fact that Comey kept meticulous notes as events unfolded would also help the prosecution's case, he said.

"In ordinary court situations, even putting aside the very different perceptions of the relative credibility of the two men, the contemporaneous documentation of the conversations at a time when Comey had no apparent reason to fabricate his report of the conversation, that makes it highly likely that his narrative is the accurate one."