Rep. Adam Kinzinger (R-Ill.) said Monday that it's difficult to know what to believe after President Trump pushed back against parts of former FBI Director James Comey's testimony.

During an interview on CNN's "New Day," Kinzinger was asked what he believes after Comey's testimony before the Senate Intelligence Committee last week and the president's response.

"It's hard to tell, and that's why we need every iteration of information that's going to include Comey. If [Attorney General Jeff] Sessions testifies tomorrow, that's going to be essential, Robert Mueller's investigation," he said, referring to special counsel Robert Mueller, now in charge of leading the federal investigation into Russian election meddling.

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"What it all comes down to is this: If you're a Republican, in some cases, every new piece of information you say, 'Well it's not real, it's not true,' " he continued. "If you're some of my friends on the other side of the aisle, every new piece of information you automatically assume is true and you scream for impeachment."

Kinzinger said ramifications of the investigation stretch beyond 2018 or 2020.

"This is about defending the entire Constitution of democracy," he said.

"And my bigger concern is we look and say we hold our leaders accountable and we need all the information to hold them accountable."

He said Comey said a lot of "concerning things" during his testimony, but he doesn't know whether Trump's actions rise to the level of obstruction of justice.

"I think there is a lot of more information that needs to come out," he said.

During Comey's testimony last week, the former FBI director said he believed he had been directed to end an investigation into former national security adviser Michael Flynn.

Trump has disputed Comey's recollection and said last week he is "100 percent" willing to give his own version under oath.