Chicago (CNN) Mark Warner, the top Democrat on the Senate Intelligence Committee, says he believes Republicans are engaged in an "unprecedented" attempt to impugn special counsel Robert Mueller's investigation and the integrity of the FBI.

The Nunes memo , which was released to the public on Friday, is the latest episode in what Warner believes is a campaign "to undermine the very validity of the Mueller investigation, and when it does appear that they may not get enough traction to fire Mueller ... undermining the integrity of the FBI and the Department of Justice," Warner told David Axelrod on "The Axe Files," a podcast produced by the University of Chicago Institute of Politics and CNN.

"We are in an area again where I can't think of any administration in my lifetime that has gone out and actively undermined law enforcement," the Virginia senator said.

Warner, who is leading the Senate investigation into Russian meddling in the 2016 election alongside Sen. Richard Burr, R-North Carolina, said neither he nor his Republican colleague was permitted to view the memo before its release after being flatly told "we will not grant you access."

Warner said he believes both he and Burr were blocked from viewing the memo prior to its release because "any legitimate entity would question both the methodology behind the memo, the willingness to use classified information this recklessly and ... feel like we need to contradict or at least need to point out its flaws."

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