Former special counsel Robert Mueller is sworn in before he testifies before the House Judiciary Committee hearing on his report on Russian election interference, on Capitol Hill, in Washington. Picture: Andrew Harnik/AP

Washington - Robert Mueller, the taciturn lawman at the centre of a polarizing American drama, bluntly dismissed President Donald Trump's claims of "total exoneration" in the federal probe of Russia's 2016 election interference. In a long day of congressional testimony, Mueller warned that Moscow's actions represented - and still represent - a great threat to American democracy.

Mueller's back-to-back Capitol Hill appearances on Wednesday, his first since wrapping his two-year Russia probe, carried the prospect of a historic climax to a rare criminal investigation into a sitting American president. But his testimony was more likely to reinforce rather than reshape hardened public opinions on impeachment and

With his terse, one-word answers, and a sometimes stilted and halting manner, Mueller made clear his desire to avoid the partisan fray and the deep political divisions roiling Congress and the country.

He condemned Trump's praise of WikiLeaks, which released Democratic emails stolen by Russia. And he said of the interference by Russians and others: "They are doing it as we sit here. And they expect to do it during the next campaign."

His report, he said, should live on after him and his team.

"We spent substantial time assuring the integrity of the report, understanding that it would be our living message to those who come after us," Mueller said. "But it also is a signal, a flag to those of us who have some responsibility in this area to exercise those responsibilities swiftly and don't let this problem continue to linger as it has over so many years."

Trump, claiming vindication despite the renewal of serious allegations, focused on his own political fortunes rather than such broader issues.

"This was a devastating day for the Democrats," he said. "The Democrats had nothing and now they have less than nothing."

Mueller was reluctant to stray beyond his lengthy written report, but that didn't stop Republicans and Democrats from laboring to extract new details.

Trump's GOP allies tried to cast the former special counsel and his prosecutors as politically motivated. They referred repeatedly to what they consider the improper opening of the investigation.

Democrats, meanwhile, sought to emphasize the most

The prosecutor who endured nearly seven hours of hearings

Mueller frequently gave single-word answers to questions, even when given opportunities to crystallize allegations of obstruction of justice against the president. He referred time and again to the wording in his report.

Was the president lying when he said he had no business ties to Russia? "I'm not going to go into the details of the report along those lines," Mueller said. Did you develop any sort of conspiracy between the Trump campaign and Russia? "Again," Mueller said, "I pass on answering."

But he was unflinching on the most-critical matters, showing flashes of personality and emotion.

In the opening minutes of the Judiciary Committee hearing, Chairman Jerrold Nadler, a New York Democrat, asked about Trump's multiple claims of vindication by the investigation.

"And what about total exoneration? Did you actually totally exonerate the president?" Nadler asked.

"No," Mueller replied.

When Rep. Adam Schiff, the Democratic chairman of the House intelligence committee, asked, "Your investigation is not a witch hunt, is it?"

"It is not a witch hunt," Mueller flatly replied.

He gave Democrats a flicker of hope when he told Rep. Ted Lieu of California that he did not charge Trump because of a Justice Department legal opinion that says sitting presidents cannot be indicted. That statement cheered Democrats who understood him to be suggesting he might have otherwise have recommended prosecution on the strength of the evidence.

But Mueller later walked back that exchange, saying: "We did not reach a determination as to whether the president committed a crime." His team, he said, never started the process of evaluating whether to charge Trump.

Though Mueller described Russian election interference as among the most serious challenges to democracy he had encountered in his decades-long career, Republicans focused on his conclusion that there was insufficient evidence to establish a criminal conspiracy between the Trump campaign and Russia.

"Those are the facts of the Mueller report. Russia meddled in the 2016 election. The president did not conspire with Russians. Nothing we hear today will change those facts," said Rep. Doug Collins, the Judiciary Committee's top Republican.

Mueller, pressed as to why he hadn't investigated a "dossier" of claims that the Republicans insist helped lead to the start of the probe, said that was not his charge. That was "outside my purview," he said repeatedly.

Mueller mostly brushed aside Republican allegations of bias, but in a moment of apparent agitation, he said he didn't think lawmakers had ever "reviewed a report that is as thorough, as fair, as consistent as the report that we have in front of us."

And when he was pressed on the fact that multiple members of his team had made contributions to Democratic candidates, Mueller bristled at the implication his prosecutors were compromised.

"I've been in this business for almost 25 years, and in those 25 years I have not had occasion, once, to ask somebody about their political affiliation," Mueller said, raising his hand for emphasis. "It is not done. What I care about is the capability of the individual to do the job and do the job quickly and seriously and with integrity."

Wednesday's first hearing before the Judiciary Committee focused on whether Trump obstructed justice by attempting to seize control of Mueller's investigation. The special counsel examined nearly a dozen episodes, including Trump's firing of FBI Director James Comey and his efforts to have Mueller himself removed, for potential obstruction.

The afternoon hearing before the House intelligence committee dove into ties between the Trump campaign and the Kremlin.

On that question, Mueller's report documented a trail of contacts between Russians and Trump associates, including a Trump Tower meeting at which the president's eldest son expected to receive dirt on Democrat Hillary Clinton.

AP