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Three months after releasing the final report on his two-year investigation into the 2016 presidential election, much of the American public remains unclear about Special Counsel Mueller's findings on whether Trump criminally obstructed justice and whether his campaign colluded with Russians.

With Trump declaring he was "exonerated" in the probe and Democrats saying it supplied ample evidence for impeachment, the notoriously taciturn Mueller will answer questions for the first time in front of two separate, Democrat-run committees in the House of Representatives.

No one was certain whether Mueller, who has resisted testifying, will go beyond the cryptic, heavily legalistic conclusions presented in his report, which allowed the president and his allies to claim the investigation was a politically charged witch hunt that found nothing of substance.

"Highly conflicted Robert Mueller should not be given another bite at the apple. In the end it will be bad for him and the phony Democrats in Congress who have done nothing but waste time on this ridiculous Witch Hunt," Trump tweeted on Monday.

"Result of the Mueller Report, NO COLLUSION, NO OBSTRUCTION!"

Democrats meanwhile hoped Mueller would make clear to the public why he did not bring charges despite damning evidence outlined in the 448-page report.

"There was enough corruption and criminality in the Trump campaign and Administration to fill a 448-page report. Every American deserves to know the crimes and corruption exposed in the Mueller Report," said Democratic Representative Ted Lieu.

Collusion, obstruction alleged

On Tuesday, Democrats were rehearsing their tactics for the marathon hearings, both to find ways to get Mueller to elaborate on his findings and to prevent Republicans on the committees from derailing the questioning.

Mueller's report documents extensive contacts between the Trump campaign and Russians, including attempts to cooperate or collude, neither of which is a specific crime.

Mueller ruled in the end that there was not enough evidence to recommend charges of criminal conspiracy, the main legal charge he could make use of.

The veteran prosecutor and former FBI director also laid out in detail 10 instances when Trump allegedly tried to obstruct the investigation.

But he said he was prevented from recommending charges against Trump because Justice Department rules prohibited him from lodging criminal charges against a sitting president.

That left it to Congress to determine whether Trump committed a crime, and a minority of Democrats are pressing for the House to impeach the president.

'The report is my testimony'

How Mueller describes the obstruction allegations could make his live testimony explosive, potentially damaging Trump as he ramps up his campaign for re-election next year.

The expected six hours or more of questioning will be broadcast live nationally, allowing Americans to hear from the man behind the two-year investigation themselves.

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But Mueller, veteran of some 88 trips to Capitol Hill over a long career in government, is notorious for saying as little as possible and has already signaled that he does not want to expound on the investigation, despite the repeated attacks on his character and his work by Trump.

"Any testimony from this office would not go beyond our report," he said on May 29, as he announced the closure of the special counsel investigation.

"The work speaks for itself. And the report is my testimony."

But he also made clear he thinks the American public does not fully understand the extent and consequences of Russia's campaign to meddle in the election, which the president has repeatedly refused to acknowledge.

Trump's Justice Department meanwhile weighed in late Monday with a statement demanding Mueller stick only to what he wrote in his report, in hopes of containing any fallout from his testimony.

"It's incredibly arrogant of the department to try to instruct him on what to say," Jerry Nadler, the chairman of the House Judiciary Committee, said on CNN Tuesday.

"It's part of the ongoing coverup by the administration to keep information away from the American people."

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