US President Donald Trump has accused top officials of politicising FBI and justice department investigations to damage his Republican party.
Later on Friday, he is expected to approve the release of a memo that is thought to suggest the FBI abused its powers to spy on one of his aides.
Controversy over the memo has railed for days in US politics and the two main parties are divided on it.
Democrats say the document is aimed at derailing investigations into Mr Trump.
They portray the memo, commissioned by the Republican head of the House Intelligence Committee, Devin Nunes, as an attempt to undermine a federal inquiry into allegations that the Trump campaign colluded with Russia during the 2016 election campaign.
However, another top Republican, House Speaker Paul Ryan, played down the potential impact of the memo's publication on the inquiry led by Special Counsel Robert Mueller.
He said Congress had a duty to see surveillance powers were used correctly.
Trump 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."
The current FBI director, Christopher Wray, was nominated to the post by Mr Trump after the president dismissed his predecessor, James Comey.
Last week the agency's deputy director, Andrew McCabe, resigned. Mr Trump had repeatedly accused him of political bias but the White House said the president was not involved in the decision.BBC