Washington (CNN) Donald Trump and Robert Mueller look ready to go to war.

It was always theoretically possible that the Russia investigation could end with the President and the special counsel on either side of a constitutional showdown. But a breakneck sequence of events over the last two days may have made the nightmare scenario increasingly likely.

Trump, the rule-breaking President who relishes throwing punches, has restocked his legal bench and now has a defense team that might better reflect his aggressive tendencies.

After saying he'd love to sit down with Mueller, he now seems to be backing away. And his White House is right behind him -- branding the special counsel's probe a "witch hunt" in an official news release.

The President is also escalating the political heat around the Mueller investigation, seeking to rile up supporters, who he has been priming for months as a firewall against any eventual impeachment fight.

"There was no Collusion (it is a Hoax) and there is no Obstruction of Justice (that is a setup & trap)," Trump tweeted Wednesday.

There was no Collusion (it is a Hoax) and there is no Obstruction of Justice (that is a setup & trap). What there is is Negotiations going on with North Korea over Nuclear War, Negotiations going on with China over Trade Deficits, Negotiations on NAFTA, and much more. Witch Hunt! — Donald J. Trump (@realDonaldTrump) May 2, 2018

Mueller, the publicity-shy prosecutor and Marine combat veteran who has spent a lifetime pursuing justice, is also showing his steel.

Sources told CNN he's raised the possibility of subpoenaing the President if Trump doesn't agree to an interview -- determined to get the testimony he needs to complete his comprehensive investigation.

That's an approach that could escalate the standoff with the President into an epochal Supreme Court drama with massive political reverberations -- which many experts think Trump would lose.

"It clearly is crunch time with regards to the Mueller investigation," former Defense Secretary and CIA Director Leon Panetta told CNN's Wolf Blitzer on "The Situation Room" on Wednesday.

"I think they have reached a point where, very frankly, they are not going to be able to conclude this investigation without the testimony of the President of the United States," he said.

Hardball

JUST WATCHED Ty Cobb out in latest Trump legal team shakeup Replay More Videos ... MUST WATCH Ty Cobb out in latest Trump legal team shakeup 01:59

Trump has been frustrated for months that his legal team has counseled him against his flamethrower approach to the Mueller probe.

On Wednesday he said goodbye to Ty Cobb, an attorney who was uncomfortable with the President's combustible strategy, and replaced him with Emmet Flood, a Bill Clinton impeachment lawyer.

The move, following the arrival of Rudy Giuliani and Jane and Martin Raskin, did more than rebuff the idea that the President can't lure big legal names; it signaled a much more adversarial approach to Mueller.

Marc Mukasey, a longtime ally of Giuliani's, also is in talks to possibly join Trump's legal team, according to sources familiar with the matter, though they cautioned that no final decision has been made.

"Playing nice hasn't gotten them anywhere," a source told CNN's Sara Murray. The source also described Giuliani as a "professional assassin."

Indeed, Giuliani went after Mueller's probe as well as former FBI Director James Comey on Wednesday night in an interview with Fox News' Sean Hannity, but he also revealed that Trump had reimbursed his personal lawyer, Michael Cohen, the $130,000 in hush money that was used to pay porn actress Stormy Daniels to stay quiet about her claims of an affair with Trump a decade ago.

Trump previously has denied knowledge of the payment, which has since spurred a lawsuit against the President.

Another brass-knuckle operator, Trump's former campaign manager Corey Lewandowski, told CNN on Wednesday that Trump had tired of a team that repeatedly assured him the probe was nearly done.

"They told the President that the investigation was going to end originally in November of last year, then December and then January," Lewandowski told CNN's Jim Sciutto on "The Lead."

"It is still going, and the President obviously wants someone to come in and look at it," Lewandowski said.

Mueller's next move

JUST WATCHED Cooper: Collusion questions won't go away Replay More Videos ... MUST WATCH Cooper: Collusion questions won't go away 04:07

The idea that Trump has suddenly got tough might strike some on Mueller's team as ironic given the months of vitriolic tweets aimed their way.

But if the President is dedicated to a more adversarial posture, it is unlikely to faze their relentless boss. Sources told CNN that in a meeting with the President's lawyers, Mueller had raised the prospect of compelling Trump's testimony if he did not voluntarily agree to talk.

Michael Hayden, who ran the CIA and the National Security Agency when Mueller helmed the FBI, said the special counsel's character dictated that he eliminate every lead.

"One of the things I think Bob feels as if he has got to do when we get to the end of this dark cloud hanging over us, no matter which way it goes, he needs to be able to say, 'I have been exhaustive,' " Hayden said. "I think Director Mueller really wants to make sure he pulls over all rocks."

Just because Mueller is determined to get Trump's testimony, it does not necessarily mean that he thinks the President is guilty of a crime or an impeachable offense.

Prosecutors often use interviews with people involved in an investigation to rule out theories or close off lines of inquiry. Mueller's willingness to obtain a subpoena could also be a bargaining chip as he tries to get the President to voluntarily sit down with him.

Given the highly sensitive political circumstances, many analysts believe that Mueller will go the extra mile to find an arrangement that accommodates the President and his lawyers.

Giuliani on Wednesday laid out a tough offer in an interview with The Washington Post , saying any encounter need be no longer than "two to three hours."

That is unlikely to satisfy Mueller, especially if the list of questions drawn up by Trump's legal team and published in The New York Times represents an accurate picture of what he wants to ask.

'The questions are an intrusion'

Given the escalating stakes, Trump's tweets on the Mueller probe Wednesday were particularly intriguing.

He tested out a legal argument that Mueller's approach violates the Constitution and a political narrative that it's a distraction from the crucial work of the presidency.

"The questions are an intrusion into the President's Article 2 powers under the Constitution to fire any Executive Branch Employee," the President tweeted, quoting a supporter, lawyer Joseph diGenova.

Speaking on the Michael Smerconish show on Sirius XM on Monday, diGenova also said Mueller would be guilty of "outrageous, sophomoric" and "juvenile" behavior if he asked what Trump was thinking when he fired FBI Director James Comey.

But to prove obstruction of justice, a prosecutor must establish that an act was carried out with corrupt intent. So in this case it would be incumbent on Mueller to understand the President's state of mind when he fired Comey.

"You have to get in his head in order to do that. That's what he is doing that is eminently reasonable, and no one can really question him for that," said Andrew Stoltmann, a prominent white-collar defense lawyer in Chicago.

Trump's claim that he is in the clear since he had constitutional authority to fire Comey does not absolve him of obstruction of justice charges, either.

"The mere fact that the President can fire someone -- and he clearly can fire the FBI director -- doesn't mean that the firing is lawful," said Stoltmann.

'This isn't some game'

JUST WATCHED WaPo: Mueller floated possible Trump subpoena Replay More Videos ... MUST WATCH WaPo: Mueller floated possible Trump subpoena 01:24

Many of Trump's supporters have long wished he would quit commenting on his legal situation -- because he often seems to dig himself in deeper.

They often cite the example of Clinton, who repeatedly stressed how hard he was working for the American people in winning the battle of public opinion and surviving impeachment.

Trump did step up his efforts to combat his legal vulnerability with a political weapon.

He quoted one of his former lawyers in a conversation with Mueller reported by The Washington Post.

"This isn't some game. You are screwing with the work of the president of the United States." John Dowd, March 2018. With North Korea, China, the Middle East and so much more, there is not much time to be thinking about this, especially since there was no Russian "Collusion." — Donald J. Trump (@realDonaldTrump) May 2, 2018

" 'This isn't some game. You are screwing with the work of the president of the United States.' John Dowd, March 2018," Trump tweeted. "With North Korea, China, the Middle East and so much more, there is not much time to be thinking about this, especially since there was no Russian 'Collusion.' "

Trump's complaint that he is too busy to testify will likely not cut much ice legally: In 1997 the Supreme Court ruled that Clinton's argument that he was too busy to give testimony in the Paula Jones civil suit had no merit.