(CNN) Attorney General Jeff Sessions needs to reveal what he knows specifically about the meetings between President Donald Trump and former FBI Director James Comey, a Republican congressman on the foreign affairs committee said Monday.

Sessions will testify at a public hearing of the Senate intelligence committee Tuesday afternoon, the committee said in a statement.

"We need to talk about the Russia meetings, the meetings with the ambassadors," Rep. Adam Kinzinger told CNN's Kate Bolduan on "At This Hour." "What exactly does he know about the discussion between Comey and President Trump?"

Following Comey's public testimony last Thursday, he told senators in a closed hearing that afternoon that Sessions may have had a third interaction with Russia's ambassador to the US, according to people familiar with the briefing.

As part of his role on the foreign affairs committee, Kinzinger said he meets with a lot of ambassadors and doesn't always remember whom he met. But he hopes Sessions can be more transparent.

"Right now you have people that are stuck in a way that anytime new information comes out, 'let's assume it's all fake news,'" he said. "And there are some that immediately yell, 'Impeachment!'"

"I don't think we finish those whole conversations until we just know everything, so I just want more information about everything that can come out," the Illinois lawmaker said.

It is difficult to determine who is being truthful between Trump and Comey, Kinzinger said.

"James Comey told his story to the Senate. The President says it is not true," CNN's John Berman said earlier Monday on "New Day." "Who do you believe?"

"It is hard to tell," Kinzinger replied. "That's why we need every iteration of information."

"What it comes down to is this: If you are Republican, in some cases, every new piece of information you say, 'It is not real. It is not true,'" the Illinois lawmaker said. "If you are some of my friends on the other side of the aisle, every new piece of information you automatically assume is true and scream for impeachment."

But Kinzinger, who served in the Air Force in Iraq and Afghanistan, said getting to the bottom of this investigation is about more than partisanship.

"This, for me, is about (something) bigger than what it means for 2018, what this means for 2020 and what this means -- this is defending the institution of democracy," he said.

And Trump's frequent commentary on matters such as the investigation into alleged ties between his campaign and Russia are distracting from his agenda and hurting the GOP, Kinzinger added.

"I wish he would spend his efforts messaging these issues instead of, again, the day-to-day of this investigation," he said. "It doesn't serve him well or the party well to comment on everything."

After avoiding Twitter during Comey's testimony before the Senate last week, Trump has repeatedly slammed the former FBI director.

"I believe the James Comey leaks will be far more prevalent than anyone ever thought possible. Totally illegal? Very 'cowardly," he tweeted Sunday.

I believe the James Comey leaks will be far more prevalent than anyone ever thought possible. Totally illegal? Very 'cowardly!' — Donald J. Trump (@realDonaldTrump) June 11, 2017

Former White House press secretary Ari Fleischer advised Trump to "stop talking" Sunday before he finds himself in legal trouble.

"Advice 4 POTUS: You have not been vindicated. U won't be unless Bob Mueller says so. Stop talking. You're heading into a giant perjury trap," tweeted Fleischer, who worked in George W. Bush's White House.

Advice 4 POTUS: You have not been vindicated. U won't be unless Bob Mueller says so. Stop talking. You're heading into a giant perjury trap. — Ari Fleischer (@AriFleischer) June 11, 2017

Kinzinger, who declined to endorse Trump during last year's election and has criticized him before for not staying on message as President, said it would best if Trump allows the investigation into Russia's role in the 2016 presidential election to move forward.

"I think the President needs to let the investigation move on," the Illinois congressman said. "We will come to answers in the investigation. It may take a while. Hopefully, sooner than later."