Donald Trump has accused bosses at the FBI and Justice Department of 'politicising' investigations in favour of the Democrats.
It comes as he was set to approve the release of a controversial memo which is likely to inflame tensions further between the President and the bureau.
The confidential memo, drafted by the Republicans, accuses the FBI of abusive surveillance tactics in its investigation into alleged links between his White House campaign team and Russia.
The four-page document is also said to allege FBI misconduct in the initial stages of its inquiry.
But the bureau said it has "grave concerns about material omissions of fact" in the memo, which could be released on Friday.
It said the document is misleading and could jeopardise national security.
The President claimed leading investigators were biased against his Republican party.
He tweeted: "The top Leadership and Investigators of the FBI and the Justice Department have politicized the sacred investigative process in favor of Democrats and against Republicans – something which would have been unthinkable just a short time ago."
He added: "Rank & File are great people!"
Democrats and critics in the intelligence community say the planned release of the memo is a stunt.
They claim it uses very selective information that cannot be countered publicly without revealing more secrets about government intelligence operations.
They say the ultimate goal of the memo, with the President's support, is to undermine special counsel Robert Mueller's investigation into possible collusion between the 2016 Trump campaign and Moscow.
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There is speculation that if the document is released, it could lead to high-level resignations in US law enforcement agencies, including FBI director Christopher Wray.
He was appointed by Mr Trump after the President fired predecessor James Comey.