(CNN) Democrats say a whistleblower who has been a key component of a Republican-led investigation that's targeted Hillary Clinton's role in the approval a uranium sale to a Russian state energy company provided Congress with no evidence of wrongdoing involving Clinton, President Bill Clinton or the Clinton Foundation.

And they say the Justice Department confirmed to Congress that the whistleblower, William Campbell, was an FBI informant who had credibility issues as a witness that ultimately led the DOJ to pursue other charges in a corruption case against a Russian energy official.

Campbell was interviewed last month by staff of the House Intelligence, House Oversight and Senate Judiciary Committees over what he knew about Russian influence in the Obama administration's decision to approve the 2010 sale of Canadian uranium mining company Uranium One to Russia's Atomic Energy Agency, Rosatom.

The deal had to be approved by the Committee on Foreign Investment in the United States, a committee that comprises representatives from several US government agencies, including the State Department, which at the time was led by Hillary Clinton.

The Democratic pushback against the whistleblower, which was released Thursday in a memo summarizing Campbell's interview, is the most aggressive attempt yet by Democrats to undercut the Republican allegations of wrongdoing in the Uranium One case. Democrats made some similar claims in a letter last month from Rep. Elijah Cummings and Rep. Adam Schiff to two GOP chairmen of those committees and requested a transcribed interview with Campbell.

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