After a drumbeat of anticipation, a secret memo was released Friday that U.S. President Donald Trump's allies in the media and Congress had predicted would set off a bombshell, exploding the entire foundation of the Russia investigation.

Spoiler alert: Not quite.

What the just-declassified memo does is allege that a key source in the Russia affair hated Trump, and that federal officials central to the probe against him played fast and loose with surveillance protocols.

The four-page memo was written by Devin Nunes, a Republican congressman from California and the chair of the House intelligence committee. The memo centres on allegations of impropriety in how federal officials obtained the right to monitor Trump foreign-policy adviser Carter Page.

It alleges that:

British ex-spy Christopher Steele detested Trump and was desperate to stop him from being president. At the time, it says Steele was being paid $160,000 US "to obtain derogatory information on Donald Trump's ties to Russia" by a firm hired to dig up dirt on behalf of the Hillary Clinton campaign.

Steele talked to the media while also acting as an FBI informant. During the 2016 election, he communicated concerns — first to Yahoo News, then Mother Jones magazine — that Trump was compromised. The memo says he misled the FBI about his contacts with journalists.

The FBI later dropped Steele as a source: "Steele's numerous encounters with the media violated the cardinal rule of source handling — maintaining confidentiality — and demonstrated that Steele had become a less-than-reliable source for the FBI."

The FBI had leaned heavily on Steele's tips to renew a warrant to spy on Trump adviser Carter Page. The warrant was renewed in October 2016. Federal officials had previously monitored Page as early as 2013, amid concern he was a Russian asset.

Federal officials did not disclose the partisan payments to Steele when they sought the warrant in national-security court.

The officials should have disclosed that Associate Deputy Attorney General Bruce Ohr's wife "was employed by Fusion GPS [the company that hired Steele] to assist in the cultivation of opposition research on Trump."

Several senior officials signed off on the warrant request, with one key name on that list: Rod Rosenstein.

He is now the top Justice Department official in charge of the Russia investigation. If he were fired and replaced with someone more hostile to the probe, his successor could either end the investigation or set new rules to constrain it.

Trump was asked whether he wanted to fire Rosenstein and replied: "You figure that one out."

But the document unveiled Friday fell short of the more hyperbolic previews offered by Trump's staunchest allies.

One memorable example was Fox News prime-time host Sean Hannity, who predicted it would be worse than Watergate and would end the Mueller investigation by proving that it rested on a faulty foundation.

"Your witch hunt is now over," Hannity declared on his show a few days ago.

But there's a weakness in the argument that the document fatally undermines the Russia probe.

It's pointed out in the document itself, in the final paragraph on the final page. The document confirms that the FBI counter-intelligence probe into the Trump campaign ties with Russia began in July — three months before the events described in the memo.

Christopher Steele, a former British intelligence officer, compiled an explosive and unproven dossier on Trump's purported activities in Russia. (Victoria Jones/PA via Associated Press)

It says the FBI probe started in July 2016 after receiving a tip that another Trump aide, George Papadopoulos, told an Australian diplomat in May 2016 that Russia had political dirt on Democratic presidential candidate Hillary Clinton, information Australian officials passed to the U.S. government, the New York Times reported in December.

Papadopoulos has since pleaded guilty to lying to the FBI, and is now co-operating with the Mueller probe.

Democrats dismissed the document as a dud, at best. At worst, they cast it as a dishonest work of partisan origami that cuts around inconvenient facts that remain classified and are central to the case.