Washington (CNN) Donald Trump's behavior isn't doing much to bolster White House assurances that he's got nothing to worry about from Robert Mueller's probe, after a series of potentially ominous turns in the Russia investigation.

The President's recent barrage of tweets and comments and testimony from sources close to him -- coinciding with thickening intrigue around the special counsel -- hint instead at deep concern on Trump's part.

"While the disgusting Fake News is doing everything within their power not to report it that way, at least 3 major players are intimating that the Angry Mueller Gang of Dems is viciously telling witnesses to lie about facts & they will get relief. This is our Joseph McCarthy Era!" Trump tweeted Wednesday, a day after blasting the special counsel as a "conflicted prosecutor gone rogue."

Despite this outburst of fury, White House press secretary Sarah Sanders painted a portrait of a President who was serenely awaiting Mueller's findings.

"I don't think the President has any concerns about the report because he knows that there was no wrongdoing by him and that there was no collusion," Sanders told reporters at her first daily briefing in a month.

The explanation for Trump's angst over his predicament seems to lie in a flurry of startling and potentially significant developments and reports swirling around his jailed ex-campaign chairman Paul Manafort and other associates.

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Trump, the most powerful man in the world who crafted a self-flattering image as the ultimate strongman boss, is in a deeply vulnerable spot and appears to feel cornered and in increasing peril.

He has no choice but to watch as Mueller, an adversary whose discreet public profile makes him an elusive target, grinds away, apparently getting ever closer to Trump's inner circle and perhaps even to the President himself.

"The Mueller investigation is what it is. It just goes on and on and on," Trump told The Washington Post Tuesday when asked about his relentless, unseen foe.

Trump's lawyer Rudy Giuliani offered a hint of the toll being inflicted on Trump behind the scenes when he told CNN's Dana Bash the President had been "upset for weeks" about "the un-American, horrible treatment of Manafort."

Details emerge

While the White House refuses to budge from its denial of collusion between Trump and Russia in 2016, a string of events now coming to light is stretching the notion that this is all one harmless coincidence to the limit of credulity.

In the space of a few days, it emerged that Manafort's cooperation deal collapsed after Mueller accused him of lying on multiple occasions.

The Guardian reported on Tuesday that Manafort met Wikileaks founder Julian Assange, who is accused of blasting out emails stolen by Russian spies to attack Hillary Clinton, on a number of occasions in the Ecuadorian embassy in London. Both Manafort and Wikileaks have issued strenuous denials of the report -- Manafort called it "totally false and deliberately libelous" and Wikileaks was "willing to bet the Guardian a million dollars and its editor's head that Manafort never met Assange."

The paper's sourcing was also vague, making it difficult to assess the veracity of the reporting. So for now, joining the dots cannot produce definitive conclusions.

CNN later obtained draft court documents Tuesday with which Mueller plans to show how Trump associate Roger Stone allegedly sought information and emails from Wikileaks using another operative, Jerome Corsi, as a go between.

In another development, CNN contributor Carl Bernstein reported Tuesday that Mueller's team has been investigating a meeting between Manafort and Ecuadorian President Lenin Moreno in Quito in 2017 and has specifically asked if WikiLeaks or Assange was discussed in the meeting, according to a source with personal knowledge of the matter.

Such is the secrecy around Mueller's investigation that no one outside knows what he knows. The special counsel has yet to lay out any case against Trump or his close associates on either alleged collusion or obstruction of justice.

If there is such supporting evidence, it is not clear that Mueller would be able to prove malicious intent by Trump. But all of the recent developments suggest that the special counsel has penetrated deeply into the events surrounding the troubled 2016 election.

In his dismissal of a cooperation agreement with Manafort , for instance, Mueller's team said it has evidence to prove the former lobbyist lied "on a variety of subject matters" -- a fact that alone must leave Trump wondering about the extent of his knowledge.

There was immediate speculation that Manafort's apparent attempts to mislead Mueller amounted to an implicit plea for a pardon from the President. The White House said there had been no talk of such a step. But adding to speculation that Manafort is angling for a pardon, Giuliani told Bash that the two legal teams had been in contact. This also raised the possibility that Trump has a genuine window into Mueller's progress, another factor that might explain his recent anger.

The New York Times reported that the cooperation between the two legal teams had inflamed tensions between the special counsel and Manafort's lawyers.

Potential impact

Former Watergate special prosecutor Richard Ben-Veniste told CNN's Brooke Baldwin on Tuesday that if the report of Manafort-Assange meetings was correct it would be extraordinary.

"What in the world would Mr. Manafort be doing meeting with Julian Assange if not to talk about Assange's role as a cut out for disseminating information the Russians obtained by illegal hacking?" Ben-Veniste said.

It was not clear whether Manafort's alleged lies dealt with the reported meeting with Assange. But given that the fugitive Australian is confined to his hideaway in the embassy, he or his hosts are likely under surveillance, intelligence that Mueller would likely be able to access.

Mueller is not the only potential threat to the President interested in the deepening questions. Democrats, poised to take power in the House, are already gearing up to reinvigorate a congressional investigation on Russia.

"A meeting with Julian Assange would be the smoking gun," said Rep. David Cicilline, D-Rhode Island, who sits on the House Judiciary Committee.

The House Democratic majority could eventually pose a grave threat to Trump's presidency and represents a devastating erosion of the force field of political protection so far offered by the GOP majority on Washington power -- another possible reason for his mood.

In the long term, any sustained dip in Trump's popularity -- his disapproval rating climbed to 60% in Gallup's weekly tracking poll -- could even eventually raise questions about the solidity of his standing among Senate Republicans that has so far always been seen as impenetrable.

Expectations high

The latest drama around Manafort is even more tantalizing given the possibility that the collapse of the cooperation agreement could prompt Mueller to reveal more about his tightly private investigation.

Special counsel prosecutors plan to detail Manafort's alleged lies in a number of areas in a sentencing memo that could be filed with the court in the coming weeks.

Mueller has used indictments and court filings throughout his tenure to embroider a rich picture of Russian intelligence hacking, a social media campaign to disrupt the election and cozy ties between Manafort and pro-Russian political figures in Ukraine.

So expectations will be high ahead of his filing if it is done in public

"Without releasing a report, without another indictment, Mr. Mueller would be in a position to reveal a lot of information that we would all find very interesting about his investigation in open court," said Ben-Veniste.

Such a filing is also now seen in Washington as a potential way around any attempt to disrupt a final report of the Russia investigation by Trump's acting-Attorney General Matt Whitaker.

The new focus on the man picked to succeed the sacked Jeff Sessions may also point to another possible spur for the President's current fury.

Trump has often seemed to know more about the probe than is available, and it's possible that Whitaker, who is now in charge of overseeing Mueller, has read him in on the inside story of the investigation.

CLARIFICATION: This story has been updated to make clear the White House has said there has been no talk of Trump pardoning Manafort.