The US House Intelligence Committee's top Democrat has accused Republicans of amending a memo about claims of FBI surveillance at the 2016 election.
Adam Schiff said Republicans had changed the text after it was voted on.
The secret document was passed by the Republican-dominated committee but needs approval from President Donald Trump to be made public.
It is believed to accuse the FBI of abusing its surveillance powers to target the Trump campaign.
The four-page document was compiled by staffers for House Intelligence Committee head Devin Nunes, a member of Mr Trump's Republican party.
Concerns about its contents were raised by the FBI itself which complained of "material omissions of fact that fundamentally impact the memo's accuracy".
Democrats fear the document may be an attempt to discredit the inquiry into Trump campaign links to Russia, which is being led by Special Counsel Robert Mueller.
If Mr Trump gives his approval, the memo may be released later on Thursday.
What do we know about the memo's contents?
It apparently accuses the Department of Justice (DoJ) and the FBI (Federal Bureau of Investigation) of abusing a programme known as the Foreign Intelligence Surveillance Act (Fisa) during the presidential election campaign.
The allegation is that the FBI spied on a member of Mr Trump's campaign on the basis of unproven accusations against Mr Trump known as the "Russian dossier".
That dossier was compiled by former UK intelligence agent Christopher Steele with money financed in part from Hillary Clinton's campaign.
Following his State of the Union speech on Tuesday night, Mr Trump was heard telling a Republican lawmaker he was "100%" for releasing the document.
Were the changes to the memo significant?
According to Mr Schiff, the text approved by the committee on Monday is not the same as that given to the White House.
BREAKING: Discovered late tonight that Chairman Nunes made material changes to the memo he sent to White House – changes not approved by the Committee. White House therefore reviewing a document the Committee has not approved for release. pic.twitter.com/llhQK9L7l6— Adam Schiff (@RepAdamSchiff) February 1, 2018
Mr Nunes sent a version of the memo that was "materially altered", the Democrat said. He gave no details.
Quoted by Reuters news agency, a spokesman for Mr Nunes described the changes to the memo as minor, including two edits requested by the FBI and Democrats themselves.
Mr Schiff's allegation was, the spokesman added, an "increasingly strange attempt to thwart publication".
What does the FBI say?
In a rare statement, the agency said it had had "limited opportunity" to review the document before the committee voted to release it on Monday.
"We are committed to working with the appropriate oversight entities to ensure the continuing integrity of the Fisa process," it added.
The DoJ has said it would be "extraordinarily reckless" to release the memo.
How are relations between Mr Trump and the FBI?
Trump officials say the memo proves his allegation that he has been treated unfairly by the FBI.
Devin Nunes, who served on the Trump team during his White House transition, described the FBI's objections as "spurious".
After firing FBI director James Comey last year, Mr Trump reportedly asked his temporary replacement, Andrew McCabe, how he had voted in the 2016 presidential election.
Mr McCabe resigned last month as Mr Trump accused him of pro-Democratic bias. He had been planning to retire in March.
In December, Mr Trump reportedly challenged Deputy Attorney General Rod Rosenstein, who appointed Mr Mueller, over his loyalties. He wanted to know whether Mr Rosenstein was "on my team", CNN reports.
Is this really about undermining the Mueller inquiry?
Mr Schiff suggested the White House would use the memo, if released, to fire Mr Mueller or Mr Rosenstein.
"This is not about the facts," he said. "This is about a narrative the chairman [Mr Nunes] wants to put out - a misleading narrative, to undermine the FBI, undermine the [justice] department and ultimately undermine Bob Mueller."
However, Mr Nunes said it was clear that top officials had used "unverified information in a court document to fuel a counter-intelligence investigation during an American political campaign".
"Once the truth gets out, we can begin taking steps to ensure our intelligence agencies and courts are never misused like this again," he added.