Nadler's committee views the full Mueller report as vital to its own corruption and obstruction of justice investigation of Trump. McGahn defied a subpoena for the documents Tuesday after the White House directed him not to comply.
Barr has released a redacted version of Special Counsel Robert Mueller's report here on his investigation into Russian interference in the 2016 US presidential election.
He ultimately reached no conclusion on whether Trump had criminally obstructed justice, citing in part an opinion from the department's Office of Legal Counsel that says a sitting president can not be indicted.
The letter, which says it was signed by former federal prosecutors who served in both Republican and Democratic administrations, specifically addresses a number of instances that were detailed in Special Counsel Robert Mueller's report where the president tried to curtail the investigation.
Under the procedure set by the House panel, the committee's majority and minority counsel would have questioned Barr for 30 minutes each.
The judiciary committee is scheduled to vote Wednesday on whether to hold Barr in contempt for not complying with a subpoena for the unredacted report and failing to show up at a committee hearing last week.
President Donald Trump stops to speak to reporters as he prepared to board Marine One on the South Lawn of the White House on January 19, 2019, in Washington, DC.
The special counsel did not make a decision about whether President Trump obstructed justice and, following that, Attorney General William Barr concluded there was insufficient evidence to support obstruction charges.
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Hundreds of former federal prosecutors (439 on Monday evening)- Democrats and Republicans - have signed a statement countering Attorney General William Barr's testimony to the U.S. Senate last week that there was not sufficient evidence to say U.S. President Donald Trump had obstructed justice with regard to Special Counsel Robert Mueller's probe into Russian meddling in the U.S. 2016 presidential elections.
Democrats told reporters on Tuesday that McConnell is sending proposals to crack down on interference to a "graveyard" that ignores Mueller's report and its findings that Russians meddled in the 2016 presidential election. He called the Senate majority leader "shameful" and said "the entire Republican Party now is going off the cliff once and for all".
"In terms of what you said earlier, Ken Starr, I want to ask you about - you said Bob Mueller deserves the sharpest criticism for sandbagging Bill Barr. Congress should now interview Mueller and his senior staff", he added. "His director of intelligence said that they're getting bolder", she said on CNN's "State of the Union".
While a contempt vote would send a message, it wouldn't force the Justice Department to hand over the report, nor would it guarantee criminal charges against Barr.
"But the members of our committee need to see the full report and the supporting documents so we can continue to do our work [and] conduct oversight in a sober and responsible way".
In his letter, Mnuchin wrote: "In reliance on the advice of the Department of Justice, I have determined that the Committee's request lacks a legitimate legislative goal, and. the Department is therefore not authorized to disclose the requested returns and return information".
Democratic House leaders could also file a lawsuit against the Justice Department to obtain the Mueller report, an option that could take months or even years to resolve.
However, just as Attorney General Barr's memo did not fully exonerate the president, other prosecutors are pointing out that this statement doesn't constitute the be-all, end-all proof of guilt that some might be hoping for.