House Republicans are prevailing in their political battle to discredit special counsel Robert Mueller's federal Russia investigation, according to an adviser who has served both Democratic and GOP-led administrations.

"I must say I think that they have succeeded beyond their dreams in discrediting Mueller," David Gergen, an adviser to former Presidents Richard Nixon, Gerald Ford, Ronald Reagan, and Bill Clinton, said Thursday during an interview on CNN.

"And I think that's one of the reasons why they want to keep plowing down this path," Gergen continued, referring to congressional probes into possible wrongdoing at the Justice Department and the FBI. "It's not pretty, but its working to some degree in the sense that a lot of Republicans have turned against Mueller now."

Gergen, who is now a political pundit on CNN, added that the upcoming 2018 midterm elections would traditionally be a referendum on Trump, but GOP lawmakers on Capitol Hill were turning it into a referendum on Mueller and "what he's trying to do."

Public opinion of Mueller dropped to an all-time low in June, according to a Politico/Morning Consult poll, as President Trump and his allies continue to attack the former FBI director, who is investigating possible collusion between Trump’s 2016 campaign and Russia.

The poll showed 53 percent of respondents held a negative view of the special counsel, who himself is a Republican, resulting in a 26 percentage point drop compared to a year earlier when the same poll was conducted.

A group of high-profile GOP congressmen have clashed with DOJ and FBI officials in recent months as they pursue documents covering the Russia probe — including the FBI's use of informants to make contact with the 2016 Trump campaign and speculation it had abused Foreign Intelligence Surveillance Act powers to gather information on Trump associates — and its examination of Hillary Clinton's private email server.