(CNN) The Justice Department on Thursday quietly published on its website some never-before-seen internal legal opinions that could help President Donald Trump block congressional requests as he faces impeachment by the US House and a trial in the Senate.

Eight of the opinions appear to bolster the White House's stonewalling of Congress on witness testimony and document subpoenas. The opinions date back to the 1970s, when President Richard Nixon faced impeachment, and the early 1980s. One from 1982 was written by the former head of the Office of Legal Counsel at the request of Rudy Giuliani, who at the time worked within the Justice Department.

Some of the opinions appear to have been made public before, and some have only been cited by the Justice Department in other legal arguments. Those released Thursday hadn't all been collected before on the Justice Department's central website regarding its internal legal opinions.

A Justice Department official acknowledging the releases said these opinions were cited in the Office of Legal Counsel's more recent opinion that former Trump White House counsel Don MGahn should be immune from subpoenaed congressional testimony. The House has sued for McGahn to testify, winning at the trial court stage, and the Justice Department is appealing. The newly released opinions were requested by the House as part of the McGahn lawsuit, according to the official.

The collection could be a valuable central resource for the President in the coming weeks, fleshing out the authority the executive branch has given itself to ignore congressional requests.

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