President Donald Trump Sunday intensified his attacks on Robert Mueller's Russia investigation as biased against him, but stopped short of targeting the special counsel - whose ouster lawmakers warned would cross a "massive red line" and could spell the "beginning of the end" for the president.

In an early-morning flurry on Twitter, Mr Trump insisted that Mr 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!" Mr Trump wrote. The night before, he had tweeted, "The Mueller probe should never have been started."

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! — Donald J. Trump (@realDonaldTrump) March 18, 2018

The salvo was the latest in a growing confrontation over Mr Mueller's investigation, which is probing whether Mr Trump's campaign colluded with Russian attempts to sway the 2016 election in his favour, but also whether the president sought to obstruct justice.

Mr Trump also took aim at former FBI deputy director Andrew McCabe, who had been in the president's line of fire and was sacked on Friday, and former FBI director James Comey who was sacked by Mr Trump last year over the Russia probe.

It remains unclear when Mr Mueller will conclude his investigation, but as it moves closer to the president, his inner circle and his business interests, he has lashed out ever more sharply.

A 'violation'

Mr Trump had warned last July that if Mr Mueller broadened his inquiry to include the finances of the Trump Organisation, it would be a "violation."

But Mr Mueller's team has now subpoenaed Russia-related documents from the president's business, the New York Times reported on Thursday.

Until now, the president has steered clear of directly attacking Mr 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 organised crime.

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Republican allies of the president have seized upon the fact 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 Mr Mueller defenders say he, and more broadly the FBI, have safeguards in place to avoid letting any investigation be tainted by politics.

Mr 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.

'Beginning of the end'

Lawmakers, including heavyweights from Mr Trump's own Republican Party, issued pointed warnings Sunday that any attempt by the president to oust Mr Mueller as special counsel would have grave consequences.

"He is following the evidence where it takes him and I think it's very important he be allowed to do his job, without interference," Republican Senator Lindsey Graham said on CNN. "And there are many Republicans who share my view."

Any move to oust Mr Mueller, Mr Graham said, "would be the beginning of the end of his presidency. Because we're a rule of law nation."

AshLee Strong, a spokeswoman for Republican House of Representatives Speaker Paul Ryan, said: “As the speaker has always said, Mr. Mueller and his team should be able to do their job.”

In an effort to tamp down the chatter, White House lawyer Ty Cobb issued a statement on Sunday night saying Mr Trump was not weighing Mr Mueller’s removal.

“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.

Representative Adam Schiff, the top Democrat on the House Intelligence Committee, told ABC that firing Mr Mueller "would undoubtedly result in a constitutional crisis."

In earlier tweets, Mr Trump accused Mr Comey of lying under oath in congressional testimony and dismissed as "fake memos" contemporaneous notes Mr McCabe and Mr Comey took of their interactions with the president.

Those memos could be fodder for Mr Mueller's probe regarding potential obstruction of justice by the US leader.

After reports that Mr Mueller had the memos, Mr Trump tweeted, "Spent very little time with Andrew McCabe, but he never took notes when he was with me. I don't believe he made memos except to help his own agenda, probably at a later date. Same with lying James Comey."

Mr McCabe, who was sacked two days before he had been due to retire, has said he is the victim of a Trump administration "war" against the FBI and the special counsel.

Mr Comey has pushed back as well. "Mr. President, the American people will hear my story very soon. And they can judge for themselves who is honorable and who is not," tweeted the former FBI chief, whose highly anticipated memoir is set for release a month from now.