(CNN) Robert Mueller's long-awaited public testimony next month will give Democrats their best and perhaps last chance to seize on the Russia scandal to try to inflict a decisive political wound on President Donald Trump.

The former special counsel's appearance on Capitol Hill on July 17, announced late Tuesday, represents a serious blow to a President who has spent weeks misrepresenting Mueller's final report.

Democrats hope the spectacle of the respected former FBI director testifying on television will move Americans against Trump in a way Mueller's dense, 448-page report did not.

The appearance also has the potential to significantly reshape the debate among Democrats over whether to open impeachment hearings, a step so far opposed by House Speaker Nancy Pelosi, who fears a political backlash.

"Our interest is for the American people to hear it from him," House Judiciary Chairman Jerry Nadler, a New York Democrat, said on "Cuomo Prime Time."

"There has been a campaign of misrepresentation by Attorney General Barr ... by the President, who keeps saying the report found no collusion, no obstruction. That's not true either way."

Trump quickly responded to news of Mueller's looming appearance with a two-word tweet: "Presidential Harassment!"

The twin hearings, in the House Judiciary and Intelligence committees, have the potential to become one of the most riveting days of political television and congressional dramas in years.

"He commands such credibility in a town where sometimes it feels like the truth doesn't matter anymore," Rep. Raja Krishnamoorthi, an Illinois Democrat who's a member of the Intelligence Committee, told CNN's Don Lemon.

"This is Mueller unplugged. I think there is going to be a Super Bowl-sized audience for it."

Should the appearance not live up to expectations of political pyrotechnics, however, it could further help Trump's aggressive bid to shift the political narrative away from the Russia storm as he campaigns for reelection.

Democratic struggles

The breakthrough, after Mueller reluctantly signaled that he will not resist a subpoena, comes with House Democrats struggling to effectively build a public case against Trump.

Days of private hearings, transcripts of interviews of the President's associates and Democratic outrage have failed to manifestly alter baked-in public perceptions of Trump and the Russia probe -- which broadly follow partisan lines. Most polls show a majority of Americans still oppose what would be the traumatic national experience of impeachment.

But it is unclear whether the former special counsel will reveal any new information: Mueller has said he would not go beyond what he already included in his report in any testimony.

After a stellar career in law enforcement, Mueller is loath to be drawn into the kind of partisan circus that unfolds in House hearings with blockbuster witnesses facing the cameras.

"The report is my testimony," Mueller said in May.

But Democrats hope that his appearance will be a powerful moment in itself and they will seek to draw out the former special counsel -- a deeply experienced witness after decades in Washington.

They are likely to pepper him with questions like: What was the most compelling evidence of obstruction of justice on the part of the President? They will be sure to ask Mueller why he did not make a formal recommendation on whether Trump is guilty of obstruction. He will also likely to be asked to explain a letter he sent to Attorney General William Barr faulting his new superior's initial characterization of the special counsel report.

In March, Barr wrote to Congress to inform lawmakers that Mueller had not found a criminal conspiracy between the Trump campaign and Russia and that Barr and then-Deputy Attorney General Rod Rosenstein had determined there was not sufficient evidence to bring an obstruction case against the President. The move was condemned by critics as presenting a cherry-picked case and the best possible interpretation of the report for the President, which allowed Trump to set a misleading political narrative that he had been cleared by Mueller.

And Mueller may be questioned on whether he meant to hint in the report that it is up to Congress to decide whether the evidence merits impeachment hearings.

Mueller wrote in the report that his team was obliged to follow Justice Department guidance that says a sitting President cannot be indicted for a crime.

"There is no limitation on confining his testimony to the four corners of the report -- that may be his desire, but Congress has questions that go beyond the report," House Intelligence Chairman Adam Schiff, a California Democrat, told CNN's Manu Raju.

Mueller chose not to clear Trump

Mueller's only public appearance of his two-year probe -- a short press statement on May 29 -- comprehensively contradicted Trump's claims of "no collusion" with Russia or obstruction of justice.

Mueller said instead that there was "insufficient evidence" to charge a conspiracy after writing in his report that the Trump campaign "expected to benefit" electorally from Russian interference.

On the question of obstruction of justice, Mueller added that "if we had had confidence that the President clearly did not commit a crime, we would have said so."

Republicans are certain to use the hearing to run interference for the President, as they have during other Democratic-led hearings seeking to highlight the Russia scandal.

They are likely to point out that Mueller did not recommend prosecution of the President or members of his campaign team over their mysterious meetings with Russians during the campaign, which were detailed in the report.

Republicans will enter the hearing armed with Trump's talking points, including his claim that the special counsel's team of prosecutors was packed with Democrats who were biased against the President.

"I hope the special counsel's testimony marks an end to the political gamesmanship that Judiciary Democrats have pursued at great cost to taxpayers," said Rep. Doug Collins of Georgia, the top Republican on the Judiciary Committee.

It was not clear whether the White House or the Justice Department can do anything to curtail Mueller's testimony. He is no longer employed by the Justice Department.

Pelosi, who must control a growing minority of around 80 House members who want Trump impeached, suggested the hearings would require members of Congress to honor their oath and "patriotic duty to follow the facts, so we can protect our democracy."

In the absence of impeachment hearings, which would not likely be followed by a conviction of Trump in the Republican-led Senate, Pelosi hopes to use public cross-examinations of key witnesses in the Russia saga to convince Americans that Trump is unfit to serve a second term.