WASHINGTON (AFP) - Two weeks after he exonerated President Donald Trump in the Russia meddling investigation, Attorney-General Bill Barr faces mounting pressure to show the full evidence behind his decision.

Allegations that the US Justice Chief downplayed serious evidence of illegal obstruction by Mr Trump in Special Counsel Robert Mueller's final report are fuelling demands that he release the entire, unexpurgated document to Congress.

News reports, citing unnamed members of Mr Mueller's staff, said Mr Barr ignored the summaries that Mr Mueller's team prepared for public release, and, instead, issued his own on March 24, in which he peremptorily cleared the president of any wrongdoing.

And Mr Barr now says he will not release key evidence given to Mr Mueller's grand jury, a special panel used by prosecutors in politically-sensitive cases. Democrats suspect the evidence could be damning to the president - setting up a legal and political showdown.

The Democrat-led House Judiciary Committee this week prepared to subpoena the full report, a move that Mr Barr and the White House will almost certainly contest.

And last Thursday, committee chairman Jerry Nadler demanded Mr Barr turn over all communications between his office and Mr Mueller's, following the reports that Mr Mueller's staff were unhappy with the way Mr Barr presented their conclusions.

Mr Barr's distillation "appears to minimise the implications of the report as to the president", said Mr Nadler. "Releasing the summaries - without delay - would begin to allow the American people to judge the facts for themselves."

At stake is the president's ability to put the Russia probe behind him and look to 2020 for re-election.

Mr Trump, who declared a "complete and total exoneration" when Mr Barr announced Mr Mueller's conclusions, said that Democrats "are fighting hard to keep the Witch Hunt alive".

"This is the highest level of Presidential Harassment in the history of our Country!" he tweeted.

On Tuesday (April 9), members of Congress might get their first chance to press Mr Barr in public about the Mueller report, when he appears before the House Appropriations Committee in a hearing nominally focused on the Justice Department Budget.

In his four-page summation of the 22-month investigation on March 24, Mr Barr said that Mr Mueller found no evidence of collusion between Mr Trump's campaign and Russia in the 2016 election, and that there was insufficient evidence to charge Mr Trump with obstruction.

Yet Mr Barr also conceded that Mr Mueller did compile evidence of obstruction, and quoted the special counsel as saying that "while this report does not conclude that the president committed a crime, it also does not exonerate him".

Suspicions are deep among Democrats that Mr Barr took advantage of his position to clear Mr Trump and now wants to keep the most damaging parts of the report secret to protect the White House.

Mr Trump chose the veteran Republican lawyer to lead the Justice Department after firing his predecessor Jeff Sessions, whom the president resented for recusing himself from oversight of the Russia probe.

In June 2018, with Mr Sessions' job already known to be imperilled, Mr Barr sent an unsolicited legal memo to the Justice Department and White House, strongly criticising the Mueller investigation and its impingement on the president's prerogatives.

Then a Washington corporate attorney - but without knowledge of the internal work of the Russia probe - Mr Barr declared that "Mueller's obstruction theory is fatally misconceived" and based on "a novel and legally unsupportable reading of the law".

The memo only came to light in December after Mr Trump had sacked Mr Sessions and chosen Mr Barr to replace him.

So far, Mr Barr has held firm to his stance that he will release this month more of the Mueller report, but stripped of evidence and testimony given to Mr Mueller's grand jury.

The grand jury material is essential. As is done frequently in such high-profile cases, Mr Mueller used the panel of citizens to develop and hear the evidence and depose key witnesses behind closed doors.

But that evidence is usually kept secret by law unless a prosecutor ultimately decides to level charges in the case.

Democrats argue that Congress, in its constitutional responsibility to enforce the law against the president, has the right to review any evidence against him of wrongdoing, even that of a grand jury.

Last Friday, the Justice Department, commenting on a fresh ruling in a case on grand jury secrecy, signalled its stance against that.

"The Department of Justice will continue to defend the long established tradition of protecting grand jury information," said spokesman Kelly Laco.