High-profile supporters of Donald Trump have begun attacking the independent investigator looking into the Russia affair, raising the question of whether the president could indeed attempt to fire him.

The sound of sharpening knives echoed everywhere Monday.

Several Trump-supporting media figures called for Robert Mueller to be fired; a Trump friend said the president was actually considering it; a Trump lawyer wouldn't rule it out; a conservative newspaper suggested Mueller faces a conflict of interest; and a top Trump surrogate accused him of partisan bias.

A Trump friend who was spotted visiting the White House on Monday later told a PBS interviewer that the president was indeed mulling over firing Mueller. Online news mogul Chris Ruddy said: "I think he's weighing that option... I personally think it would be a very significant mistake."

Others sounded more enthusiastic about dropping the axe. Newt Gingrich, who is close to Trump, tweeted: "Republicans are delusional if they think the special counsel is going to be fair. Look who he is hiring. Check (political donation) reports. Time to rethink."

Gingrich is correct that there is a certain partisan tilt to the team of top-notch investigators being assembled by the ex-FBI director: Jeannie Rhee, Michael Dreeben, Andrew Weissmann and James Quarles are renowned in their fields and have made repeated donations to Democrats.

Firebrand pundit Ann Coulter urged the government to fire Mueller. She said the counsel was now pointless, as it's become clear Trump wasn't an initial target of the Russia investigation, which may now be expanding into new areas.

Another talk-radio Trump booster, Laura Ingraham, who was rumoured to have been considered for a White House position, directed her followers to a story in a conservative newspaper that suggests Mueller might be tainted.

The piece in the Washington Examiner points out his years-long friendship with a key witness, whose testimony will be especially important if the probe morphs into an obstruction-of-justice investigation into the president. That witness: James Comey, who replaced Mueller as FBI director.

A senior political writer for the newspaper sounded the alarm in a piece titled, "Is Robert Mueller conflicted in Trump probe?" It put the question to five lawyers who shared their assessments, anonymously. Two called the friendship with Comey improper, but manageable; two called it serious; one called it a non-issue.

This apparent effort to lay the groundwork for Mueller's firing raises a key question: Could the president do it?

The answer is yes.

So says someone who helped craft the special-counsel law. Neal Katyal spelled out three ways to thwart this kind of criminal probe in a Washington Post piece titled, "Trump or Congress can still block Robert Mueller. I know. I wrote the rules."

He explained how reforms were prompted by the never-ending, ever-expanding probe into Bill Clinton, which began with an examination of land trades in Arkansas and ended in an impeachment and national drama over Monica Lewinsky's blue dress.

The government let the old independent-counsel law expire in 1999 — at that point even Clinton's tormentor, investigator Kenneth Starr, suggested the law needed to change. Policy-makers created new limits on the investigator's power and placed it under the accountability of the government.

Enter the current law.

"First, most simply, Trump could order Mueller fired," Katyal said. He said the president could ask his attorney general or, because Jeff Sessions has recused himself, ask his deputy attorney general, a civil servant, Rod Rosenstein, to do it. He said Trump could even order the special-counsel regulations repealed and fire Mueller himself.

"Either of those actions was unthinkable to us back in 1999 ... At the same time, after Trump's firing of ... Comey ... many things once thought beyond the realm of possibility look less so now."

Second, he said Congress could muck up Mueller's investigation. It could give immunity to witnesses at public hearings, increasing the chance that subsequent court cases collapse as prosecutors struggle to prove they didn't get any help from the exempted testimony.

Third, he said, Rosenstein could also re-define the scope and powers of the investigation.

A writer on the legal blog Lawfare said the president could even argue that the counsel is impeding his constitutional power over foreign affairs. In fact, Trump has already argued that all these controversies are hampering his ability to repair relations with Russia.

On the other hand, it would cause a political storm, said the writer.

"Such an action would incur a severe political cost," wrote Josh Blackman, associate professor at the South Texas College of Law Houston. "The termination of Mueller would amount to an admission of guilt and obstruction of justice. The fallout from the firing of Mueller would likely be as explosive as (Richard Nixon's) firing of special counsel Archibald Cox in 1973."

One of Trump's lawyers refused to rule out a firing.

Asked about it on ABC this weekend, Jay Sekulow said that if there were a serious basis for it, the president would be allowed: "Whether he would do it is ultimately a decision the president makes."

A poll last month by Harvard-Harris suggested 75 per cent of Americans wanted a special prosecutor. That appeared to include Gingrich, who until recently expressed support for Mueller's appointment on the grounds that it would alleviate pressure in the Russia affair.

Given that kind of support, a Democratic lawmaker, Ted Lieu, taunted the White House over the Mueller-firing chatter: "Double dare exists. But is there such a thing as an 'infinite dare'? If so, I infinite dare (Trump) to fire the special counsel," he tweeted.

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