The Democratic party has released a counter memo which defends the investigation into alleged links between Donald Trump's campaign and Russian officials.
The partially redacted memo, released by Congress on Saturday after weeks of suppression by the White House, hits back at Republican claims of political bias in the FBI Russia probe.
But Republicans say Democrats failed to declare that research in the document that supported a surveillance warrant on a former adviser to Mr Trump, Carter Page, was funded by Hillary Clinton's campaign and the Democratic National Convention.
They argue that is a serious breach of the rules that politics should be kept out of the investigation.
The claims concern a document by British spy Christopher Steele, which is now known as the Trump-Russia dossier. It was used in a government application to monitor Mr Page's activity during the presidential race.
A memo detailing Republican concerns was released two weeks ago by Mr Trump, amid strong objections by the FBI over its accuracy.
Democrat Congressman Adam Schiff said the Saturday release of the counter memo should "put to rest any concerns", and show that there was no evidence of "illegal, unethical, or unprofessional behaviour" by the FBI, the Justice Department and the court that issued a warrant to investigate Mr Page.
The document shows that when requesting to put Mr Page under surveillance, the Justice Department made it clear that Mr Steele's research was funded by a political party looking to discredit Donald Trump.
But it did not name the Clinton campaign, and Mr Trump's camp has insisted this is not good enough.
:: Russia "laughing their asses off" at US, Donald Trump says
Dem Memo: FBI did not disclose who the clients were – the Clinton Campaign and the DNC. Wow!
— Donald J. Trump (@realDonaldTrump) 24 February 2018
Sarah Sanders said the document "fails to answer serious concerns" about the use of partisan opposition research that was used by one candidate to place another under surveillance.
Critics of the White House have called Republican accusations over the memos an attempt to distract from the ongoing investigation into Russia.
The President initially blocked the Democratic memo countering the claims, but it was released with significant redactions following FBI negotiations.
As well as details on the funding for Mr Steele's research, it included information about alleged Russian meetings with Mr Page, Russian attempts to groom him as a spy, and a 2016 FBI interview with him as a "person of interest".
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Mr Trump has said the Republican memo "vindicates" him – but other Republicans have urged that the spat over the documents should not undermine trust in the ongoing investigation into alleged links between his campaign and Russian officials.
The probe into Russian interference in the US presidential elections has seen special counsel Robert Mueller charge nearly 20 people, including former national security adviser and former campaign aide Nick Gates.
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