Sally Yates, an Obama administration holdover fired by Trump as acting attorney general after she defied his refugee ban, described the approach as "boiling the frog slowly" -- increasing the heat incrementally so it's not noticed until too late.

"I can remember a time when he would issue a tweet or directive and the reports would be, 'In an unprecedented act, the president did X or Y,"' Yates said May 21 on MSNBC. "It's not so unprecedented anymore, and oftentimes it doesn't even make it through the full 24-hour news cycle."

Mueller has already secured guilty pleas from five people and filed charges against 14 others and three companies. The far-reaching investigation has reached people close to Trump and his businesses, including Michael Cohen, his personal lawyer and "fixer."

Trump's barrage of tweets and comments are backed up by allies such as Rudy Giuliani, who was brought onto the president's legal team in mid-April. The former New York mayor has echoed Trump in declaring the probe a "witch hunt" led by angry Democrats, and repeatedly attacking the Justice Department he once served as a US Attorney.

"We're more convinced, as we see it, that this is a rigged investigation," Giuliani said Sunday on CNN's "State of the Nation," one of two appearances for the day on political talk shows. Asked whether he sees the probe as legitimate, Giuliani, replied: "Not any more. I don't. I did when I came in."

In one of his most recent twists, Trump demanded a Justice Department investigation into whether the FBI or the DOJ "infiltrated or surveilled the Trump campaign," prompting a briefing last week with members of Congress on documents related to Mueller's probe -- including the name of an informant the Federal Bureau of Investigations used as it opened a counterintelligence investigation into Trump's campaign in 2016.

Trump is "simply trying to de-legitimise the Mueller investigation, the FBI, the Department of Justice," Michael Hayden, who led the Central Intelligence Agency and the National Security Agency under Republican and Democratic presidents, said Sunday.

"He's willing to throw almost anything against the wall," Hayden said on ABC's "This Week," mentioning Trump's unsubstantiated claims about wiretapping, "unmasking" US citizens in surveillance operations, and abusing warrants. "And for a lot of people in the country, one or another or many of those things have already stuck."

The number of Americans who think Mueller's investigation is politically motivated has grown to 53 percent in early May from 48 percent in December, according to a CBS News poll conducted May 3 through May 6.

Flake said there's concern that Trump is laying the groundwork to fire Mueller or Deputy Attorney General Rod Rosenstein, and if that were to happen, "obviously, that would cause a constitutional crisis."

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Yet Republican lawmakers who warned in the past that firing Mueller would be a "red line" Trump can't cross were mostly silent when, for example, the president started criticising the special counsel by name in Twitter messages in April. Talk of bipartisan legislation to protect Mueller, discussed in late 2017, has gone silent.

Congress is "complicit" with Trump when Republican allies such as House Intelligence Chairman Devin Nunes and a "weak" Speaker Paul Ryan refuse to stand up for the independence of the Justice Department, Representative Adam Schiff, the panel's top Democrat, said on Sunday.

"That means that the rule of law is now on shaky ground," Schiff said on ABC.

Schiff, who attended the briefing last week on documents related to Mueller's probe, said there's no evidence to support the president's theory that the FBI was spying on his campaign -- a theory that Flake called "a diversion tactic."

"This is just a piece of propaganda the president wants to put out and repeat," Schiff said on ABC. "And certainly we've seen this pattern before."

Democratic Senator Chris Coons of Delaware said that despite talk from Trump and Giuliani about a "rigged" investigation by angry Democrats, Mueller, Rosenstein and many others involved with the Justice Department are "lifelong Republicans."

"To continue to undermine the credibility of the FBI and the Department of Justice in this way does not serve the interest of law enforcement or the rule of law," Coons, a member of the Senate Judiciary Committee, said on "Fox News Sunday."

Bloomberg