(CNN) Donald Trump -- warning that his presidency is being stolen -- sure seems threatened by Robert Mueller, especially considering his declaration that the special counsel handed him "complete and total exoneration."

Trump on Sunday called for Mueller to be barred from testifying to Congress -- in a hearing that could come as soon as May 15 -- as his anxiety exploded in a double barreled tweet that reversed a position expressed only two days earlier.

"After spending more than $35,000,000 over a two year period, interviewing 500 people, using 18 Trump Hating Angry Democrats & 49 FBI Agents - all culminating in a more than 400 page Report showing NO COLLUSION - why would the Democrats in Congress now need Robert Mueller to testify.......," Trump wrote Sunday.

After spending more than $35,000,000 over a two year period, interviewing 500 people, using 18 Trump Hating Angry Democrats & 49 FBI Agents - all culminating in a more than 400 page Report showing NO COLLUSION - why would the Democrats in Congress now need Robert Mueller.......

"Are they looking for a redo because they hated seeing the strong NO COLLUSION conclusion? There was no crime, except on the other side (incredibly not covered in the Report), and NO OBSTRUCTION. Bob Mueller should not testify. No redos for the Dems!"

The President spent months shaping the aftermath of the Russia investigation, so clearly he has no desire to see his narrative contradicted by Mueller breaking his public silence.

That's especially the case since it emerged last week that Mueller wrote to William Barr to register concern about the attorney general's public presentation of his work — that clearly favored Trump — in a letter to Congress in March.

"Despite the tremendous success that I have had as President, including perhaps the greatest ECONOMY and most successful first two years of any President in history, they have stollen two years of my (our) Presidency (Collusion Delusion) that we will never be able to get back.....," Trump added in a later tweet.

(Trump later corrected his spelling, saying that two years of his presidency had been "stolen.")

Despite the tremendous success that I have had as President, including perhaps the greatest ECONOMY and most successful first two years of any President in history, they have stollen two years of my (our) Presidency (Collusion Delusion) that we will never be able to get back..... — Donald J. Trump (@realDonaldTrump) May 5, 2019

Sunday's Twitter blasts came as Trump's surreal capacity to ride out scandals and controversies that would puncture the viability of any other politician faces tough new tests.

The IRS has promised an answer on Monday for Treasury officials to respond to Democrats demanding tax returns that candidate Trump refused to reveal in a buckling of political norms.

Barr meanwhile has until 9 a.m. ET, to reply to a demand from a Democratic committee chairman to provide the un-redacted Mueller report or face the prospect of contempt proceedings.

The tidal wave of challenges reflects a building constitutional showdown in Washington as the White House seeks to thwart a Democratic oversight war on multiple fronts.

That the coincidence of several strands of scandal whirling around the President on Monday feels just like a new normal rather than an aberration -- is in itself an eloquent commentary on the Trump era.

Trump's public message to Barr

The White House and Democrats are locked in a struggle to define the end game of the Mueller report even though -- for now at least -- it seems unlikely to lead to an impeachment crisis.

Trump's tweets looked a lot like a public message to Barr -- who critics say rolled out Mueller's report in a way that was beneficial to the President.

Barr told lawmakers last week he would not object to Mueller testifying about the report that did not establish a conspiracy between Trump's campaign and Russia, but also did not provide a prosecutorial ruling on whether Trump obstructed justice.

Barr stepped into the vacuum and decided there was no obstruction case for the President to answer, even though Mueller provided evidence that appear to point to presidential wrongdoing.

On Friday, Trump seemed unperturbed when asked by reporters in the Oval Office whether Mueller should testify.

"I don't know. That's up to our attorney general, who I think has done a fantastic job," Trump said.

A weekend of reflection and signs that House Democrats are close to securing an appearance by Mueller appear to have changed Trump's mind.

It is not clear what would happen if Barr goes back on his willingness to sign off on a Mueller hearing that was made most recently under oath in the Senate Judiciary Committee last week.

Mueller has shown over a lifetime in public service that he respects authority and chains of command -- so he could chose to defer to the attorney general if he blocks testimony.

But it is also clear that he is uneasy with how his report has been interpreted. And as a private citizen once his employment as special counsel ends, he might feel compelled to set the record straight in public.

If he doesn't show up voluntarily, Democrats are likely to issue a subpoena in order to require the former FBI director to appear.

But even if Mueller does agree to talk, the extent of his testimony could be limited by deference to procedures on grand jury secrecy and the need to protect classified information. His reticence to protect ongoing investigations, which the Justice Department cited as a major reason for redactions in the Mueller report could also be a factor.

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The prospect of Mueller testimony -- that has so far been theoretical -- became real, if only for a short time on Sunday.

House Judiciary Committee member Rep. David Cicilline, a Rhode Island Democrat, said there was a deal for Mueller to appear on May 15, in what would be a dramatic day even by Washington standards.

"We think the American people have a right to hear directly from him," Cicilline said on "Fox News Sunday." "Obviously until the day comes, we never have an absolute guarantee."

A few hours later, Cicilline said nothing had been agreed to yet.

"That's the date the Committee has proposed, and we hope the Special Counsel will agree to it. Sorry for the confusion," Cicilline wrote on Twitter after reports on his earlier comments spread.

Peter Carr, a spokesman for the special counsel, declined to comment.

Schumer: Mueller testimony 'imperative'

Senate Minority Leader Chuck Schumer seized on rising expectations of a Mueller hearing, apparently trying to make it more politically difficult for the White House to back away.

"We ought to hear the full testimony from Mueller," Schumer said at a news conference Sunday. "Given the fact that Barr does not seem to be a neutral observer here, Mueller's testimony is all the more important."

As a day of rain kept him off the golf course on Sunday, the President went on a tweeting binge, highlighting comments by political allies that supported his position on Mueller.

Intense pressure is building in Washington as the White House seeks to move on from the Mueller investigation and parries Democratic investigations and oversight demands.

The President signaled on Friday that he was still deciding whether to let former White House counsel Don McGahn appear before Congress. McGahn was a key witness to behavior by the President that the special counsel appeared to suggest in his report could amount to obstruction of justice.

Trump is suing to stop his own accountants from handing over his financial records. The Treasury has blown past two deadlines for the handover of the President's tax returns to another House committee.

The President and his children have also lodged another lawsuit to try to block the release of records from two banks to another House committee.

Increasingly, the separation of powers showdown looks like it will spark a legal imbroglio that will take months to work its way through the courts — a scenario that might suit the White House with Trump's 2020 re-election campaign getting under way.

With that race in mind, House Speaker Nancy Pelosi delivered an extraordinary warning in a New York Times interview published Saturday.

She told the paper that Democrats needed to win in 2020 by a sufficiently large margin that Trump would find it impossible to challenge the result.

"We have to inoculate against that, we have to be prepared for that," Pelosi told the newspaper.

Pelosi told the Times she had worried that Trump would "poison the public mind" and "challenge each of the races" if Democrats didn't win by an overwhelming majority in 2018.