Congressional Republicans are seeking to make the case that the FBI's investigation into President Trump's campaign and possible collusion with Russia was based on flawed or politically tainted evidence connected to partisans loyal to Hillary Clinton.

The House Intelligence Committee memo spearheaded by Rep. Devin Nunes (R-Calif.) that is the talk of Washington will be at the center of the argument. The release of that four-page memo is expected as early as Friday.

Another document - an eight-page criminal referral filed with the Justice Department by Senate Judiciary Committee Chairman Chuck Grassley (R-Iowa) and Sen. Lindsey Graham (R-S.C.) - is also part of the GOP case.

While the FBI has been protesting the release of the Nunes memo, it has been working behind the scenes to vet a version of the Grassley-Graham memo, which is expected to be released in redacted form soon. The FBI is also seeking redactions to the Nunes memo, though it is not clear the White House or congressional Republicans will agree to them.

Republicans believe both documents will back up arguments that evidence used to justify the FBI's probe came from partisans loyal to Clinton, sources said. They are also expected to play into arguments from some Republicans that special counsel Robert Mueller's probe into Russia is based on false information.

Those suggestions have provoked a backlash from Democrats. Some Republicans have also expressed concerns, with GOP leaders saying Mueller should be allowed to continue his work. Speaker Paul Ryan (R-Wis.) on Tuesday backed releasing the Nunes memo but also offered support for Deputy Attorney General Rod Rosenstein.

The GOP's arguments that the FBI investigated on flimsy evidence will zero in on ex-British intelligence operative Christopher Steele, whose unverified dossier of Trump allegations gave enormous momentum to the probe during the bitterly fought 2016 presidential election, say sources familiar with the memo.

They will argue the FBI failed to critically assess the political motives and credibility of Steele and did not fully disclose that evidence came from Clinton supporters as it sought to get permission from courts for surveillance warrants.

"The fact that half to three-quarters of the evidence the FBI used to unleash the most awesome of surveillance powers upon Donald Trump's inner circle came from sources tied directly to his Democratic opponent should worry us all, especially when that happened during an election," said one senior Republican directly familiar with the evidence, describing the party's core concerns.

"The FBI allowed itself to be used by Clinton partisans to parlay single-sourced, mostly unverified evidence into a counterintelligence probe with clear weaknesses that weren't disclosed," the source added.

Sources in both parties and in law enforcement say the emergence of Steele and his dossier in the summer of 2016 gave the FBI the jolt to open a full counterintelligence probe into alleged Trump-Russia collusion.

Steele, a British intelligence officer for decades, had provided reliable evidence that helped the FBI in a prior foreign corruption case and his early package of information contained multiple allegations of Trump-Russia collusion organized and sourced liked real raw intelligence, sources said.

So the FBI put great credence in Steele's work product when it approached the courts for legal surveillance authority, even though much of what Steele provided could not be immediately corroborated, the sources said.

Steele was employed by the research firm Fusion GPS, which was in turn being paid to do opposition research on Trump by the Clinton campaign and the Democratic National Committee (DNC).

Fusion GPS was initially hired by the Washington Free Beacon, a conservative website backed in part by Republican donor Paul Singer, to dig into Trump. As Trump was clinching the GOP nomination, the Free Beacon told Fusion GPS to stop doing research. Fusion GPS was then hired by Clinton and the DNC through the law firm Perkins Coie.

Republican investigators say they have evidence that Steele first approached the FBI with his allegations on July 5, 2016, the same day then-FBI Director James Comey announced he would not pursue criminal charges against Clinton for passing more than 100 classified documents through her private email server.

Republicans believe the date of Steele's approach to the FBI is evidence of politics, since his employer was being paid by a Clinton campaign that for months was angered by the bureau's probe of the email controversy.

Republican investigators say it is unclear exactly when the FBI learned that Steele was being paid by Clinton's campaign. But by late July 2016, just weeks after he first contacted the FBI, the formal counterintelligence probe was opened, the sources said.

After Steele came forward, the FBI received information from a friendly foreign government that also pointed to possible links between Trump's campaign and Russia.

The diplomat from Australia heard a Trump campaign aide named George Papadopoulos boasting in a bar that Russia was considering the release of damaging emails from Clinton during the election. The diplomat actually heard the conversation in May 2016 but it did not get reported to U.S. authorities until weeks later, after Steele had begun cooperating with the FBI, the sources said.

The FBI gave credence to the information both because it came from a trusted foreign source and because emails hacked from the Democratic Party and Clinton campaign chairman John Podesta had already leaked onto the internet, the sources said. The FBI already suspected Russia was involved in those hacking attacks by the time the information was reported by the Australian diplomat, the sources said.

As the investigation was unfolding, Steele also forwarded the FBI a new piece of evidence with substantially similar information as that contained in his dossier. The FBI learned that information came from a private investigator with longtime ties to the Clinton inner circle, according to sources familiar with the evidence.

The sources declined to provide the private investigator's name, though British papers have suggested a Clinton supporter named Cody Shearer may have offered Steele information about Trump and Russia.

Republican investigators say they have developed evidence that Steele broke off his relationship with the FBI shortly before Election Day 2016 as he and his employer, Fusion GPS, began talking to reporters.

The investigators say the bumpy ending was due in part to the fact that Steele and Fusion GPS were upset the FBI suddenly reopened the Hillary Clinton email case but did not seem as invested in the Trump-Russia case. Many Democrats believe the FBI's reopening of the Clinton investigation cost her the election.

Glenn Simpson, the co-founder of Fusion GPS who hired Steele, acknowledged to the House Intelligence Committee in an interview that he and Steele began talking to reporters because they were angry about the reopening of the Clinton email case.

"We decided that if James Comey wasn't going to tell people about this investigation ... we would only be fair if the world knew that both candidates were under FBI investigation," Simpson testified.

Simpson separately told the Senate Judiciary Committee that Steele ended his FBI relationship because he was alarmed by a story in The New York Times on Oct. 31, 2016, that claimed the FBI had looked into ties between the Trump campaign and Russia but found nothing.

"Chris severed his relationship with the FBI out of concern that he didn't know what was happening inside the FBI and there was a concern that the FBI was being manipulated for political ends by the Trump people," Simpson said.

According to the Republican investigator, the FBI also developed concerns that either Steele or his employer, Fusion GPS, were contacting media organizations to disseminate the very intelligence that was at the heart of its probe, the sources said.

Simpson, in his testimony, said he and Steele had conducted off-the-record briefings with reporters in the fall of 2016, but claimed none of the memos written by Steele were shown or given to the journalists who attended. The purpose of the briefings, he said, was to encourage reporters to ask questions about whether the FBI was in fact investigating ties between the Trump campaign and Russia.

Republicans plan to portray the timing of Steele's cooperation, its abrupt ending, his connections to the private investigator, and the Clinton and DNC payments as prima facie evidence of politically motivated digging rather than high-quality intelligence worthy of the FBI's extensive investigation, the sources said.

Grassley referred Steele to the Justice Department for contacts he and Fusion GPS had with the media around the same time he was cooperating with the FBI. Grassley has asked the Department of Justice to investigate whether Steele concealed those contacts from the FBI or whether the FBI knew about the contacts and did not properly disclose them to Congress or the courts.

The reason the media contacts raised red flags for both Senate and House GOP investigators is that the FBI used a fall 2016 news story with allegations of Trump-Russia collusion that were similar to Steele's dossier as evidence to ask the Foreign Intelligence Surveillance Court to issue a surveillance warrant against Trump campaign adviser Carter Page, the sources said.

The sources declined to identify the specific story. An Oct. 31, 2016, story by reporter David Corn in Mother Jones, however, is the first known story to make mention of Steele's work.

The Steele dossier, the PI report and the news story "all seem like independent corroborating evidence" but Republicans now believe they were "the fruits of a single politically poisoned tree planted by Clinton partisans," said a source familiar with concerns that are raised in the House Republican Intelligence panel memo.

"Rather than quality double-sourced intelligence, it is our conclusion it was nothing more than thinly veiled, single-sourced political opposition research masquerading as intel," the source said.