Attorney General Jeff Sessions. Win McNamee/Getty Images

President George W. Bush's top ethics lawyer told Business Insider on Thursday that while it was "debatable" whether Attorney General Jeff Sessions perjured himself during his Senate confirmation hearing, he should still resign.

Richard Painter, a professor at the University of Minnesota Law School who was the chief White House ethics lawyer from 2005 to 2007, added that the latest Russia-related White House firestorm could leave Sessions open to the risk of "blackmailing."

"The thing with Sessions is that the Russians almost certainly have a recording of these conversations or detailed notes about the conversations," he said.

Painter continued: "And so, they've got something over Sessions. Sessions will be asked what was said in these conversations. And if that doesn't match what the Russians have in their records ... then they've got him, and they have this over him for the rest of his term. We have no idea of knowing whether we're in that situation, but it's just way too high a risk."

With what is now known about Sessions' conversations with Russian Ambassador Sergey Kislyak, Painter said he didn't think the attorney general "could effectively run that department."

Wednesday-night reports about two conversations between Sessions and Kislyak sparked an uproar, with numerous Democratic politicians calling for the attorney general to resign, while others, including prominent Republicans, called for him to recuse himself from any investigation involving President Donald Trump's ties to Russia or the country's influence in the 2016 presidential election. In a Thursday press conference, Sessions announced he would recuse himself from any investigation that involved the Trump campaign.

Sessions told the Senate Judiciary Committee in his confirmation hearing that he had no contact with Russian officials during the campaign. He said that during an exchange with Democratic Sen. Al Franken of Minnesota.

"If there is any evidence that anyone affiliated with the Trump campaign communicated with the Russian government in the course of this campaign, what will you do?" Franken asked.

"Sen. Franken, I'm not aware of any of those activities," Sessions responded. "I have been called a surrogate at a time or two in that campaign and I did not have communications with the Russians, and I'm unable to comment on it."

Franken did not ask Sessions whether he specifically spoke with Russian operatives, but Sessions, in answering an unrelated question, said he did not have any communications with "the Russians."

Sessions' allies have insisted he did not mislead the committee and did not believe that those conversations, held in his capacity, they said, as a senior Armed Services Committee member, were relevant to the question. Opponents say Sessions may have perjured himself by not mentioning those two conversations with Kislyak.

Painter, who backed Democratic presidential nominee Hillary Clinton in the general election, said Sessions "certainly misled Congress" with that answer.

"You expose yourself to criminal prosecution by doing that," Painter said.

Painter on Wednesday night tweeted that "misleading the Senate in sworn testimony about one['s] own contacts with the Russians is a good way to go to jail."

He compared it to the circumstances that played out around Richard Kleindienst, an attorney general for President Richard Nixon. In 1972, Kleindienst, then acting attorney general, appeared before the Senate Judiciary Committee for a confirmation hearing. He was asked about White House interference in an antitrust suit against International Telephone and Telegraph Corporation and whether he ever spoke with anyone in the administration about the case.

He said he did not, which was not true, as Painter outlined in a New York Times op-ed article. When it was uncovered that Kleindienst, after being confirmed as attorney general, was not truthful, he said he believed the questions asked during his hearing were limited to a particular time period.

U.S. Attorney General Jeff Sessions holds his first meeting with heads of federal law enforcement components at the Justice Department. in Washington U.S., February 9, 2017. REUTERS/Yuri Gripas

But that argument didn't pass muster with the special prosecutor who was investigating the Watergate scandal, and Kleindienst was forced to resign. He later pleaded guilty to a misdemeanor charge of failing to provide accurate information to Congress.

Another situation that provided some parallels surrounded Bush's attorney general Alberto Gonzales, who resigned from office following a controversy over the firings of several federal prosecutors. Democratic lawmakers believed he made misleading statements about the firings before Congress, leading him to resign.

"We've already had Flynn get sacked over misleading the vice president of the United States about his contacts with the Russians," Painter told Business Insider of Michael Flynn, Trump's former national security adviser. "So this is a pretty serious situation. Whether it amounts to perjury or not, I think it's much more debatable."

"You know, I think in terms of the truth-telling involved, it falls within the same category as the Kleindienst example I discussed in the NYT op-ed," he continued, adding of President Bill Clinton: "And that you know the Clinton lying about his sex life — the difference between this and Clinton is that it's obviously so much more important than whether the president had sex with an intern, because we're talking about the Russians and spying activities within the United States, much more important. So, you know that's a factor as to whether he's fit to serve as attorney general."

During his Thursday press conference, Sessions tried to clear up some questions about his contacts with Kislyak.

In describing the meeting, of which he said he recalled little, Sessions said he discussed terrorism and Ukraine, adding that the discussion surrounding the country "got to be a little bit of a testy conversation."

On why he didn't mention the conversations during his answer in the confirmation hearing, Sessions said he was "taken aback" by the new information Franken had presented in his question.

"This allegation that a surrogate — and I had been called a surrogate for Donald Trump — had been meeting continuously with Russian officials, and that's what I — it struck me very hard, and that's what I focused my answer on," he said. "And in retrospect, I should have slowed down and said, 'But I did meet one Russian official a couple of times, and that would be the ambassador.'"

The explanation that the conversations were under the scope of Sessions' role on the Armed Services Committee, and not as a top Trump adviser, seemed off to Painter. The Washington Post reported none of the other 25 members of the committee met with Kislyak.

"I don't think it's routine to have members of the Armed Services Committee talking to the Russian ambassador," Painter said. "If I were the chairman of the committee, I wouldn't want individual senators talking to the Russian ambassador about committee business."