President Trump unleashed on special counsel Robert Mueller over the weekend, signaling a shift in his criticism of the Justice Department probe into Russian interference in the presidential election.

In a series of tweets on Saturday and Sunday, Trump took aim at Mueller, calling his investigation into possible ties between the Trump campaign and Russia a "witch hunt" and suggesting bias on the part of Mueller.

The Twitter attacks on Mueller - which follows former FBI Deputy Director Andrew McCabe's firing - were the first of their kind by Trump.

The president has frequently bashed the probe and asserted there was no collusion between his campaign and Russia, but until now Trump had never mentioned Mueller by name in his critical tweets.

According to a report by The New York Times, Trump's lawyers had asked him for months to avoid the specific mention of Mueller on the social media platform, and aides told the publication that the president is feeling more emboldened than ever to speak his mind.

The ramped-up criticism from Trump started on Saturday when his personal lawyer, John Dowd, called on Deputy Attorney General Rod Rosenstein to end the investigation on the president's behalf.

Dowd later reversed course, saying he gave the statement in his personal capacity.

"I pray that Acting Attorney General Rosenstein will follow the brilliant and courageous example of the FBI Office of Professional Responsibility and Attorney General Jeff Sessions and bring an end to alleged Russia Collusion investigation manufactured by McCabe's boss James Comey based upon a fraudulent and corrupt Dossier," Dowd said in a statement first reported by The Daily Beast.

But later the president appeared to echo Dowd's statement, saying Mueller's investigation should have never been started in the first place.

"The Mueller probe should never have been started in that there was no collusion and there was no crime. It was based on fraudulent activities and a Fake Dossier paid for by Crooked Hillary and the [Democratic National Committee], and improperly used in FISA COURT for surveillance of my campaign. WITCH HUNT!" Trump tweeted.

Trump continued to tear into the probe on Sunday morning, attacking the makeup of the investigators on the probe.

"Why does the Mueller team have 13 hardened Democrats, some big Crooked Hillary supporters, and Zero Republicans? Another Dem recently added...does anyone think this is fair? And yet, there is NO COLLUSION!"

The tweet marks a significant shift from late last year, when Trump said he believed Mueller would treat him fairly in the investigation.

"There's been no collusion. But I think he's going to be fair," Trump told The New York Times in December.

Trump's new focus on Mueller comes after the special counsel reportedly subpoenaed the Trump Organization and, according to another report, provided Trump's legal team with a series of questions leading up to a possible interview with Trump.

The president said last year that Mueller would be crossing a "red line" if his office began investigating his family's finances in other countries besides Russia.

The Mueller criticism also comes as the fallout over McCabe's firing continues to shake Washington, D.C.

Sessions fired McCabe on Friday, citing reports from the FBI's Office of Professional Responsibility and Office of the Inspector General that McCabe "made an unauthorized disclosure to the news media."

Trump praised the move in the early hours of Saturday morning as "a great day for Democracy."

McCabe fired back on Saturday, saying he never leaked any information.

"The decision to share information with the media is absolutely within my authority as deputy director," McCabe said. "I am one of three people in the FBI who has the independent authority to make that decision. People could disagree about the decisions I made ... but the fact is this is not a leak."

McCabe has also provided the special counsel with memos describing his contacts with Trump, according to multiple reports. Those memos reportedly corroborate former FBI Director James Comey's testimony about his unusual meetings with Trump in the months before he was fired.

Trump's criticism of Mueller in the wake of both McCabe and Secretary of State Rex Tillerson's firings has sparked speculation that Mueller could be next on the chopping block. But Democrats and Republicans say that would be the wrong move for Trump.

"Give him the time, the resources, the independence to do his job," House Oversight and Government Reform Committee Chairman Trey Gowdy (R-S.C.) told "Fox News Sunday" of Mueller.

Rep. Jackie Speier (D-Calif.) on Sunday warned that if Trump orders the firing of special counsel Robert Mueller, she will vote to impeach him.

"The President is careening us toward what I fear will be a constitutional crisis," Speier warned on Twitter. "Mr. President: Here is my red line-Fire Mueller and I will vote to fire you."

Another of Trump's lawyers, Ty Cobb, on Sunday issued a statement saying Trump has no plans to fire 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," the statement reads.

Updated at 8:33 p.m.