Washington (CNN) New questions hang over the integrity and motives of William Barr after it emerged Tuesday night that Robert Mueller expressed concerns about the attorney general's initial letter to Congress summarizing his special counsel report.

The bombshell revelations detonated hours before Barr opened testimony on Wednesday on Capitol Hill, an appearance that was already likely to be a political cauldron with suspicion intense among Democrats over his framing and interpretation of the Mueller investigation.

Mueller sent an initial letter on March 25 and expressed concerns the day after Barr informed Congress the special counsel did not establish a conspiracy between Trump's team and Russia and there was not enough of a case for the Justice Department to bring an obstruction case.

Mueller then wrote a second letter on March 27 -- released on Wednesday by the Justice Department -- which said "there is now public confusion about critical aspects of the results of our investigation" because of Barr's summary.

"This threatens to undermine a central purpose for which the Department appointed the Special Counsel: to assure full public confidence in the outcome of the investigations," Mueller wrote.

The special counsel criticized Barr for not fully capturing the Mueller report in its "context, nature and substance."

The news of Mueller's letter caused outrage among senior Democrats already warning that Barr's handling of a redacted version of the report was designed to produce the most favorable political narrative for Trump. Barr's decision to find that there was no obstruction case to answer also angered Trump's critics since Mueller did not come to such a conclusion in the report but did include evidence of clear attempts by Trump to thwart investigations into his campaign and his administration.

Top Democrats immediately called for a swift appearance on Capitol Hill by Mueller himself, in an escalating drama that not only hikes pressure on Barr but could influence the debate in the congressional Democratic Party about impeachment -- a step House Speaker Nancy Pelosi has been reluctant to initiate.

"The special counsel's concerns reflect our own. The Attorney General should not have taken it upon himself to describe the Special Counsel's findings in a light more favorable to the President. It was only a matter of time before the facts caught up to him," House Judiciary Chairman Jerry Nadler said in a statement.

"Attorney General Barr also should not have withheld this letter from Congress for as long as he has. I have demanded a copy from the Department of Justice. I have asked that it be delivered no later than 10:00 tomorrow morning," the New York Democrat said.

Barr is due to appear before the House Judiciary Committee on Thursday but his attendance is in doubt after he objected to Nadler's plan to include the committee legal counsels in questioning.

The attorney general is first scheduled to testify before the Senate Judiciary Committee on Wednesday at 10 a.m ET -- a session that will now take on extra significance and controversy and will have added electricity since several Democratic presidential candidates are on the panel and will be keen to make a splash.

One Democratic presidential candidate who is not on the committee -- former Housing and Urban Development Secretary Julian Castro -- tweeted Tuesday that Barr should "resign his position or face an impeachment inquiry immediately" for "willfully" misleading the American people.

New intrigue in Mueller end game

Tuesday night's stunning developments left Washington digesting an unexpected new twist in the endgame of the Mueller investigation, which, in addition to not exonerating Trump on obstruction, did not establish a conspiracy between Trump's campaign team and Russia during the 2016 election. Trump has selectively embraced Barr's narrative about the report to declare, "No collusion. No obstruction."

Phil Mudd, a former FBI agent who once worked with Mueller at the bureau and is now a CNN commentator, said Tuesday on "Cuomo Prime Time" that it was "stunning" that someone as dutiful and restrained as his former boss had written to Barr to question his summary of the special counsel's principal findings.

"This is a baseball bat wake-up call. ... You cannot underestimate what he is saying," Mudd said.

According to a source familiar with Mueller's concerns, the letter from the special counsel's office came in on March 27. It was first reviewed by Deputy Attorney General Rod Rosenstein's office and is roughly a page in length.

After reviewing it, Barr called Mueller the following day. Barr said something to the effect of "we've been friends for a long time, let's talk about this," according to a source with knowledge of the call. The call was described to CNN as polite, but there was clear disagreement.

Justice Department spokesperson Kerri Kupec said in a statement Tuesday that Mueller did not tell Barr that anything in the letter was factually wrong.

"In a cordial and professional conversation, the Special Counsel emphasized that nothing in the Attorney General's March 24 letter was inaccurate or misleading. But, he expressed frustration over the lack of context and the resulting media coverage regarding the Special Counsel's obstruction analysis," Kupec said.

Trump's lawyer Rudy Giuliani told CNN's Pamela Brown: "Mueller should have made a decision and shouldn't be complaining or whining now that he didn't get described correctly."

Televisions in the White House were tuned to Barr's appearance in the Senate Judiciary Committee. Some officials privately expressed exasperation that the Russia drama -- which haunted the administration for two years, was now suddenly back in the spotlight -- just when the West Wing hoped to move on.

Even before the new intrigue over Mueller, Barr's testimony was part of a building constitutional imbroglio over the attorney general's performance, which also includes increasing stonewalling by the White House of Democratic investigative efforts and a rush of private litigation by the President to cloak his business history.

The multiple confrontations consolidate around a common theme that is being tested in Congress and the nation's courtrooms: How much transparency does the President owe Americans?

Like everything in the Donald Trump era, the moment is tortured by angry political divides -- one reason why this debate over presidential power will be long and possibly inconclusive.

Democrats see a duty to investigate and constrain, and some even want to remove Trump over what they view as a documented record of corruption and obstruction in and out of office.

"I think the President has escalated the conflict with Congress," Rep. Jamie Raskin, a Democrat of Maryland and a member of the House Oversight Committee, told CNN. "The obstructionism that we read about the Mueller report has come galloping off the pages and right onto our front doorstep."

Republicans, eager for their own political preservation with Trump's base, view the oversight war as impeachment in disguise and an example of overreach they can exploit electorally.

"Most Americans think it's over, want to move on," Senate Majority Leader Mitch McConnell said on Tuesday.

Fight to dominate 2020 election

The strategic battle lines of the fight that will dominate Capitol Hill until the 2020 election are becoming clear.

Democrats believe there is a mountain of evidence to highlight in the Mueller report, even though the special counsel did not establish conspiracy between Trump's team and Russia and did not draw prosecutorial conclusions on obstruction.

The plurality of falsehoods came from Trump himself. Many experts also believe conduct revealed by the President in Volume Two of the report does add up to obstruction of justice.

Democrats are not just picking over ground plowed by Mueller.

They are also using the power of their House majority to discomfort the President by digging into his past personal and business finances -- a crossed red line that infuriates him.

The White House has adopted a position of maximum resistance. It has blocked testimony from key aides and slow rolled demands for documents. The Treasury Department has missed two deadlines to respond to a House committee chairman's demands for Trump's tax returns.

The accompanying political strategy with an eye on the 2020 election, is to portray Democrats as crazed by power.

"The harassment of Bill Barr and the disrespect is completely disgraceful," Trump adviser Kellyanne Conway said on Fox News on Tuesday. "I think it's outrageous they want to play the system."

'Delay, Delay, Delay'

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Ultimately, the President's strategy appears to be to mire the Democrats oversight offensive in so many procedural and legal tangles that it struggles to produce -- at least until 2020.

It's a bet that Democrats fear the political consequences of pushing for impeachment -- since they are divided on the question -- and will therefore not have sufficient leverage to hold the President to account.

That reasoning may partly explain Trump's latest gambit, a lawsuit -- along with three of his children, to stop two banks handing over financial records to congressional committees.

The suit contends that subpoenas sent to Deutsche Bank and Capital One were meant to harass Trump and to rummage and ferret through his personal life to "cause him political damage."

This is not the first time Trump restarted to a private legal strategy to thwart Democratic investigations.