WATCH LIVE: Attorney General William Barr before Senate Judiciary Committee

U.S. Attorney General William Barr (Chip Somodevilla/Getty Images)

Attorney General William Barr testifies before the Senate Judiciary Committee on Wednesday is expected to defend his actions surrounding the release of special counsel Robert Mueller's Russia report.

It emerged Tuesday night that Mueller has expressed frustration to Barr in a letter with how the conclusions of his investigation have been being portrayed.

The letter lays bare a simmering rift between the Justice Department and the special counsel about whether Barr's summary of the report adequately conveyed the gravity of Mueller's findings, particularly on the key question of obstruction.

The revelation is likely to sharpen attacks by Democrats who accuse Barr of unduly protecting the president and of spinning Mueller's conclusions in Trump's favor.