In a morning flurry on Twitter, Trump insisted that Mueller's team of investigators is staffed with "hardened" Democrats biased against him.
"Does anyone think this is fair? And yet, there is NO COLLUSION!" Trump wrote. The night before, he had tweeted, "The Mueller probe should never have been started."
The salvo was the latest in a growing confrontation over Mueller's investigation, which is probing whether Trump's campaign colluded with Russian attempts to sway the 2016 election in his favor, but also whether the president sought to obstruct justice.
It prompted warnings from several lawmakers including from Trump's own Republican party to not fire Mueller, which led by evening to a denial from a White House lawyer that the president was contemplating such a move.
Trump also took aim at former FBI deputy director Andrew McCabe, who was sacked on Friday two days before his retirement, and former FBI director James Comey, who was fired by Trump last year over the Russia probe. McCabe had an early role in the Russia inquiry and has knowledge of Trump and Comey's interactions.
McCabe's lawyer, Michael Bromwich, offered a scathing response to the attacks on his client, saying on Twitter, "We will not be responding to each childish, defamatory, disgusting & false tweet by the President... But the tweets confirm that he has corrupted the entire process that led to Mr. McCabe's termination and has rendered it illegitimate."
It remains unclear when Mueller will conclude his investigation, but as it moves steadily closer to the president, his inner circle and his business interests, he has lashed out ever more sharply.
Trump had warned last July that if Mueller broadened his inquiry to include the finances of the Trump Organization, it would be a "violation."
But Mueller's team has now subpoenaed Russia-related documents from the president's business, the New York Times reported Thursday.
Until now, the president has steered clear of directly attacking Mueller, the former FBI director who took over the Russia probe after Comey's firing May 9, 2017.
But he has taken aim at the special counsel's large team of lawyers, who were selected for their experience in financial fraud, money laundering, corruption, bribery and organized crime.
"Why does the Mueller team have 13 hardened Democrats, some big Crooked Hillary supporters, and Zero Republicans? Another Dem recently added," Trump said.
Republican allies of the president have seized upon the fact that some of Mueller's team seven out of 17 lawyers, according to The Washington Post previously donated money to Democratic political candidates.
The conservative website Daily Caller reported last month that 13 of the team were registered Democrats.
But Mueller defenders say he, and more broadly the FBI, have safeguards in place to avoid letting any investigation be tainted by politics.
Mueller, who is widely respected in both political camps, was appointed FBI chief by the Republican president George W. Bush, and kept on under the Democrat Barack Obama.
Lawmakers, including heavyweights from Trump's own Republican Party, issued pointed warnings Sunday that any attempt by the president to oust Mueller as special counsel would have grave consequences.
"Special Counsel Mueller has served our country with honesty and integrity," Republican Senator John McCain said on Twitter, insisting it is "critical he be allowed to complete a thorough investigation into Russia's interference in the 2016 election - unimpeded."
The Republican senator and frequent Trump critic Jeff Flake likewise warned on CNN that ousting Mueller was seen as "a massive red line that can't be crossed."
Representative Adam Schiff, the top Democrat on the House Intelligence Committee, told ABC that firing Mueller "would undoubtedly result in a constitutional crisis."
On Sunday evening, White House lawyer Ty Cobb denied that Trump is considering firing Mueller.
"In response to media speculation and related questions being posed to the Administration, the White House yet again confirms that the President is not considering or discussing the firing of the Special Counsel, Robert Mueller," he said.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)