Democrats have seized on the letter, arguing that it underscores their concerns that Barr has mischaracterized Mueller's findings from his two-year investigation into Russia's election meddling and the Trump campaign.

Schumer put Mueller's letter into the Senate record, meaning it will be publicly published in the Congressional Record.

Schumer added on Wednesday that the Mueller letter was a "stunning indictment" of Barr.

"What a stunning indictment of the attorney general, whose principal job in all of this was to make sure that he wasn't mischaracterizing or spinning results. This letter shows what an awful, awful attorney general Barr has been so far," Schumer said.

Schumer's remarks come as Barr is testifying before the Senate Judiciary Committee about his handling of the Mueller report.

Barr defended his handling of Mueller’s report during the hearing, saying he wanted to release the report’s bottom-line conclusions as quickly as possible because the public was in a “high state of agitation” over the results of Mueller’s investigation.