Trump's blast at a campaign rally on Friday night followed a week of rising attacks on Mueller and the FBI from pro-Trump media outlets and personalities and prominent conservatives in Congress.
The President did not name Mueller at the boisterous event in Pensacola, Florida, avoiding specific attacks on the probe after a flurry of furious tweets last weekend may have deepened his political and legal exposure.
But he enriched his building narrative that unnamed forces within the US government were thwarting his administration, just days after unloading on the FBI on Twitter, when he said the bureau's reputation was in "tatters."
Not the first time
It is not the first time that Trump has made such arguments -- he complained against the "rigged" system during last year's election in a gambit seen at the time as a face-saving hedge against a possible loss to Hillary Clinton.
But the context has changed. Trump is now the head of the government that he is accusing of conspiring against him politically. Therefore, his attacks against US government institutions, including the FBI, but which have also included the wider intelligence community and the judiciary are far more polarizing politically and risk causing long-term damage to already fragile trust in government.
They could even have constitutional implications since Trump is attacking the very system set up to constrain presidential power and to ensure integrity at the pinnacle of US government.
The timing of the assault is unlikely to be an accident.
The new intensity in attacks against Mueller and the FBI followed the plea deal reached by fired national security adviser Michael Flynn
last week, that could see him testify against key figures in the President's inner circle.
Trump responded to the rising threat by suggesting that there was something corrupt in a system that indicted Flynn but did not prosecute his former election rival over her email server.
"So General Flynn lies to the FBI and his life is destroyed, while Crooked Hillary Clinton, on that now famous FBI holiday 'interrogation' with no swearing in and no recording, lies many times...and nothing happens to her?" Trump tweeted. "Rigged system, or just a double standard?"
Trump's return to the "rigged" system rhetoric reflects his consistent political strategy of seeking enemies against which to define himself. It also plays into the suspicions of his supporters by casting himself as a outsider innocent of wrongdoing who is being persecuted by an elite establishment which has gamed Washington power for itself.
But it also has serious implications for the Mueller probe.
It's possible that the former FBI director concludes that there was no evidence of collusion by the Trump campaign during the election, and the President did not obstruct justice in the firing of his successor at the bureau James Comey.
But given the Flynn plea deal, it appears clear he has bigger targets than the former national security adviser in his sights.
In that light, attacks by Trump and the GOP on Mueller and the bureau could be an attempt to discredit any eventual conclusions that Mueller might deliver to Congress -- be they favorable or unfavorable to the President.
The simultaneous political and media campaign against Mueller, meanwhile, is raising concerns that the President has embarked on an attempt to solidify his political base and frame a political rationale for supporters in Congress to oppose any eventual move toward impeachment. The idea would be that if the system of legal accountability represented by Mueller and the FBI is "rigged" and "sick," it cannot be trusted to deliver a fair verdict on the President, a conceit that has staggering political implications.
In recent days, Trump has had new ammunition to support his claims that the Mueller probe is prejudiced against him.
Earlier this week, the special counsel removed Peter Strzok, an investigator from his team, for exchanging private messages with an FBI lawyer that could be seen as critical of Trump and supportive of Clinton.
Last year, Strzok changed a key phrase in Comey's description of how former Secretary of State Hillary Clinton handled classified information, according to US officials familiar with the matter.
The message from Trump supporters has been that Mueller's dismissal of Strzok is not a sign of a prosecutor determined to outlaw any sign of a conflict of interest, but instead evidence that the team is rotten at the core.
Mueller's close identification with the FBI, and the fired Comey has also enabled his supporters to suggest that something is not on the level.
"The vast majority of the FBI, the rank and file agents, are good people," Republican Rep. Jim Jordan told CNN's "New Day" on Friday.
"But I do think there are a lot of folks at the top who have done things that I think Americans question, members of Congress question particularly as it relates to the entire Clinton investigation and the Russian matter," he said.
Former House Speaker Newt Gingrich, who had previously praised the special counsel, said on Laura Ingraham's show on Fox News on Wednesday that "Mueller is corrupt. The senior FBI is corrupt. The system is corrupt."
Given Mueller's reputation as one of the most dedicated and by the book prosecutors of his generation, the idea that he is a latter-day J. Edgar Hoover may be a hard sell outside Trump's core political support.
After all, he stewarded the bureau through the traumatic post 9/11 period after being appointed by President George W. Bush. President Barack Obama asked him to stay on for an extra two years when he took office. He has a reputation of unimpeachable authority and quiet dignity among his peers.
But the idea that Trump could fire Mueller should be taken seriously. That's despite the possibility it could trigger turmoil and potentially resignations at the Justice Department, severe political damage to the President himself and potentially a constitutional crisis.
After all, Trump dismissed Comey despite obvious implications that he could be seen as trying to obstruct the Russia investigation. And he pardoned former Arizona Sheriff Joe Arpaio despite claims his ally violated the Constitution with his hardline immigration enforcement.
Former US attorney Preet Bharara maintained this week that it is already clearly established that the President "does not shrink from exercising his full constitutional authority."
"I would worry in a real way that Donald Trump may preemptively pardon some people, and I still worry in a real way that Donald Trump may decide to cause the firing of Robert Mueller," Bharara said on "The Axe Files" podcast
, a joint project between the Ulaniversity of Chicago and CNN.
If that is the case, Trump's remarks on look less like an attempt to fire up a Friday night crowd with red meat rhetoric and more like an attempt to prepare the political battlefield for the denouement of the Russia saga.