Washington (CNN) Democrats are pinning their last, best hopes of proving that President Donald Trump committed impeachable crimes on former special counsel Robert Mueller , who is set to reluctantly serve as a star witness in Wednesday's televised spectacle.

Mueller's date before two back-to-back House committees represents Democrats' most powerful weapon in a war for public perception over Russia that it has been losing to Trump's bruising PR campaign.

It is also shaping up as a risk for Democrats if the taciturn Mueller does not provide explosive testimony that could deal a blow to the President. The hearing seems just as likely to increase pressure from liberals on an unwilling House Speaker Nancy Pelosi to initiate impeachment hearings as it is to deflate Democrats' effort to severely damage Trump before the 2020 election

The bitter legacy of the 2016 campaign and allegations that Trump cooperated with Russia's election meddling effort and obstructed justice to cover it up have cast a shadow over the President's entire White House term and led to his mantra "No collusion," which he repeated again Monday morning.

"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.

But Mueller's nuanced report, a decision to avoid a prosecutorial recommendation and an aggressive administration effort to defuse some of his most damaging findings neutered the impact of the report he published in April

Democratic House Judiciary Committee Chairman Jerry Nadler on Sunday argued that there was "very substantial evidence" the President was guilty of "high crimes and misdemeanors" -- the Constitutional standard for impeachment.

"We have to ... let Mueller present those facts to the American people, and then see where we go from there, because the administration must be held accountable," Nadler said on "Fox News Sunday."

Trump, who has claimed he will not watch the testimony, tweeted simply on Sunday: "Presidential Harassment!"

The hearings are just the latest blockbuster political extravaganza during the turbulent and surreal Trump era and come at a politically sensitive moment. The 2020 campaign is gathering pace and the President is stoking yet another raging controversy set off by his racist tweets targeting four minority Democratic members of Congress -- part of a strategy of tearing at societal divides for his own political advantage.

JUST WATCHED Mueller: The most private public figure in Washington Replay More Videos ... MUST WATCH Mueller: The most private public figure in Washington 07:02

'Watch the miniseries'

In his report , Mueller described how the Trump campaign expected to benefit from Russian election meddling and listed multiple instances of how the President may have interfered with investigations and obstructed justice.

But Mueller's reluctance to seek the spotlight, a politically astute effort by Attorney General William Barr to manage his revelations and the complicated nature of the charges robbed the report of some of its impact.

"People don't always necessarily read the books, they don't always necessarily read the reports -- they will watch the movie, they will watch the miniseries," said Bradley Moss, a national security lawyer.

"They will watch a two or three hour televised hearing, they will watch the soundbites," Moss said on CNN's "Right Now" Friday.

The Russia controversy, despite questionable behavior by Trump, has so far lacked the dramatic era-defining moment provided by the Senate Watergate hearings in the 1970s.

A comprehensive campaign of delay and obstruction by the administration has deprived Democrats of the chance to cross examine many key witnesses such as former White House Counsel Don McGahn.

A prolonged court battle is unfolding to challenge the White House's refusal to provide evidence and testimony. Democrats could eventually prevail in those cases but each month that passes erodes the political potency of the Russia controversy and eases the President's exposure.

That's why they hope Mueller could prove to be a political game changer.

At the very least, Democrats plan to paint of picture of a criminal, ethically-bereft presidency born from a willingness to profit from a foreign power's meddling in US democracy.

While it appears impossible for Democrats to remove Trump from office given GOP control of the Senate, they hope the hearing will bolster their 2020 case that he is too corrupt and unpatriotic to deserve a second term.

Mueller does not want to testify . It took a subpoena to get him to Capitol Hill . And over years in Washington, Mueller has proven to be a witness who sticks strictly to his lane, so there is a risk his testimony could be an anti-climax.

But the special counsel did give a taste of his potential as a witness with a startling decision to break his silence and to make a public statement at the end of his investigation that dominated the news for days.

Democratic lawmakers plan to cross examine him on parts of the report that are most damning to Trump, especially actions which they believe amount to chargeable instances of obstruction.

Mueller did not conclude that Trump committed a crime in his report, telling reporters in May that there was "insufficient evidence to charge a broader conspiracy" between the Trump campaign and the Russians.

But Mueller, who brought charges against 37 people in his investigation, including six Trump associates, also pointedly did not exonerate Trump in the obstruction part of his investigation.

"If we had confidence that the President clearly did not commit a crime, we would have said that," Mueller said. That statement may provide the starting point for many of the questions from Democrats.

'A smorgasboard of obstruction'

Democrats will also hone in on the second part of the Mueller report regarding obstruction. There are five areas of the Mueller report where they think the President obstructed justice, including his efforts to fire the special counsel and to tamper with witnesses like his former campaign chairman Paul Manafort.

"There was just a smorgasbord of obstruction by Trump, whether asking staff to lie for him, whether asking staff to do deeds that were obstructive, and asking Mueller to resign, or asking (then Attorney General Jeff) Sessions to unrecuse himself, dangling pardons before Manafort, dangling a pardon before Michael Cohen at one time," Rep. Steve Cohen, D-Tennessee, said on CNN's "The Situation Room" Friday.

Another opening for Democrats lies in Mueller's admission that Justice Department guidance on the issue of prosecuting a sitting commander-in-chief meant that "charging the President with a crime was therefore not an option we could consider."

Democrats will likely try to get Mueller to say whether Trump's actions, especially in the obstruction part of the investigation, would have been charged if he was a private citizen.

The lawmakers will require discipline in their questioning and need to avoid personal histrionics in a setting in which they habitually botch cross examinations through grandstanding and lack the methodical approach of prosecutors.

Democrats on the Intelligence Committee and Republicans on the Judiciary panel have already gamed out tactics in separate mock hearings with senior aides playing Mueller , CNN reported

They will also press Mueller on the contacts with Russia and WikiLeaks detailed in the report. Some lawmakers might also wish to probe his views of Barr, after the then-special counsel wrote two letters to his boss complaining that his summary of the report did not accurately reflect his conclusions and caused public confusion.

Barr's critics warned that his intervention was a politically motivated attempt to set a misleading public narrative about the report and to shield the President.

Republican members will run interference and are likely to try to turn the hearing into a circus in a bid to convince voters it's just a typical partisan Washington mess.

They are also likely to challenge Mueller on the conduct of his team -- that Trump has complained is biased and to question the rationale for the opening of the FBI Russia investigation.

"The Republicans have not forgotten about where this investigation started, and there's going to be a lot of questions for what he did say, what he didn't say, and how this thing started," Rep. Doug Collins, the top Republican member on the House Judiciary Committee , said on Fox News on Sunday.

And Collins predicted the hearing would have little political impact overall.

"I have told some people before it's like going back and finding a book on the shelf that looks new, and then all of a sudden you begin to read it and you find out, wait, I already read this before."