The informant
Although some dismiss Trump's repeated talk of a "witch hunt" as paranoia, he has now triggered a major investigation. The Department of Justice has asked its inspector general to assess his claim in the hope of minimising further clashes between the President and federal law enforcement.
Trump's demand for an investigation followed reports by the Washington Post that a retired American professor and US intelligence source who has helped with special counsel Robert Mueller's Russia probe was in touch with three Trump advisers during the campaign.
The President and his supporters say this indicates spying, although there is no evidence to show the informant was embedded in Trump's campaign team.
The justice department was already looking into Republican complaints of FBI misconduct in the early stages of the Russia investigation.
At the request of Attorney-General Jeff Sessions and congressional Republicans, Inspector General Michael Horowitz announced a review in March into whether FBI and DOJ officials abused their surveillance powers by using a dossier compiled by former British spy Christopher Steele to justify monitoring Trump campaign adviser Carter Page.
The DOJ said it would expand the review "to include determining whether there was any impropriety or political motivation in how the FBI conducted its counter-intelligence investigation of persons suspected of involvement with the Russian agents who interfered in the 2016 presidential election".
Spokeswoman Sarah Isgur Flores added: "As always, the Inspector General will consult with the appropriate US Attorney if there is any evidence of potential criminal conduct."
Calls to expose the identity of the informant are growing. House Intelligence Committee chairman Devin Nunes last month requested documents on the source from the justice department, which refused on the grounds it could jeopardise international intelligence links and put the professor in danger.
The FBI is now working to mitigate the potential damage if the informant's name is revealed, sources told the Washington Post.
Some officials are said to be nervous the President may pull rank and force the justice department to hand over the documents on the source, which could see senior officials quit in protest.
CNN said Trump had unleashed a "political earthquake" with his demand for an investigation, more than an year after he accused Barack Obama of conducting surveillance on Trump Tower in October 2016. His claims were never proven.
The dossier
The FBI reportedly initially heard about the possible Russia link through Australian diplomat Alexander Downer, who recalled a conversation in a bar with Trump adviser George Papadopoulos.
An informant met Papadopoulos and Page because they had suspicious contacts with Russia, the New York Times reported on Saturday.
Papadopoulos was charged last year in Mueller's investigation and pleaded guilty to lying to the FBI.
But former Bill Clinton adviser Mark Penn observed in The Hill that Downer signed a $25 million contribution to the Clinton Foundation, suggesting his involvement was probably part of "opposition research".
He said the FBI lacked evidence to begin an investigation, attacking Steele's dossier as "unverified" and "fanciful", showing nothing more than "a few isolated contacts with Russians" and nothing directly related to the President.
The FBI pressed forward, Penn said, believing Trump's team "must be dirty".
It is a view shared by many people surrounding the President, who posted a Twitter tirade on Monday on the "(phony) Dossier". He quoted former Secret Service agent Dan Bongino in blaming former CIA Director John Brennan for disgracing the intelligence community with a "political hit job".
Democrats are making dark predictions over the President interfering with the justice department, warning it could have a chilling effect on federal law enforcement. The new probe could cause deep divides between the President and FBI Director Christopher Wray or Deputy Attorney-General Rod Rosenstein, who is overseeing the Mueller investigation.
Press secretary Sarah Huckabee Sanders confirmed Rosenstein and Wray would meet with the President on Monday afternoon.
"For months, @realDonaldTrump has insulted & tried to discredit the men & women at FBI & DOJ in self-serving attempts to distract from the Trump-Russia scandal," tweeted House Minority Leader Nancy Pelosi. "His conspiratorial fantasies must not be allowed to undermine the proper function of our justice system."
Senator Dianne Feinstein said the justice department was "not an arm of the White House" but was "independent and serves the American people," adding that law enforcement investigations "must be initiated and carried out free from political interference."
Adam Schiff, the senior Democrat on the House Intelligence Committee, called Mr Trump's claim of an embedded spy "nonsense" and said his demand for an investigation was "an abuse of power, and an effort to distract from his growing legal problems" that put lives and alliances at risk.
Former Director of National Intelligence James Clapper said the President's call for an investigation was "not good for the country."
The deep state
But Giuliani said the President could not be concerned about what effect an investigation may have. "To me, there's not much of a difference between an informant's ongoing collection of information in a surreptitious way or a spy," he said. "If this guy was an FBI implant into the campaign, that's as offensive as Watergate."
Trump and his supporters have ramped up their mutterings about the "Deep State" — a term for the suspected power of more permanent government structures over the elected president.
Their conspiracy theories are not isolated. Rumours of an influential "7th Floor Group" or "Shadow Government" pulling strings in Washington persist on both sides of American politics.
But to claim the FBI is trying to destroy Trump may be a stretch. After all, FBI director James Comey's odd decision to reopen an investigation into Hillary Clinton's emails just before the election played into Trump's hands, and the Russia link was not mentioned until afterwards — although the President went on to fire the Bureau chief.
The Financial Times compared Trump to Kenneth Williams' Julius Caesar from Carry On Cleo, uttering the words: "Infamy, infamy, they've all got it in for me."
The President's main concern now is ensuring he does not, like Hillary Clinton, fall victim to the ill-timed release of an investigation's results.
Trump has been asked to provide an interview for Mueller's probe in July ahead of a report date of September, according to Giuliani.
"We said to them, 'If we're going to be interviewed in July, how much time until the report gets issued?" the lawyer and former New York mayor told the Associated Press on Sunday
"They said September, which is good for everyone, because no one wants this to drag into the midterms."
The President has indicated he will give the interview, although Giuliani said a decision would not be made until after the North Korea summit slated for June 12.
The stand-off has begun.
