US President-elect Donald Trump would have been convicted for his alleged effort to overturn the result of the 2020 election if he hadn't been elected four years later, said a report by then special counsel Jack Smith published by US media early Tuesday.

The US Department of Justice's "view that the Constitution prohibits the continued indictment and prosecution of a President is categorical and does not turn on the gravity of the crimes charged, the strength of the Government's proof, or the merits of the prosecution, which the Office stands fully behind," the report said.

"Indeed, but for Mr. Trump's election and imminent return to the Presidency, the (Special Counsel's) Office assessed that the admissible evidence was sufficient to obtain and sustain a conviction at trial."

Trump, who returns to the White House on January 20, had been accused of conspiracy to defraud the United States and conspiracy to obstruct an official proceeding -- the session of Congress called to certify President Joe Biden's election win but which was violently attacked on January 6, 2021 by a mob of the Republican's supporters.

Smith, who was special counsel appointed to investigate Trump, dropped the federal criminal case against the incoming leader after he won November's presidential election.

Soon after the report's overnight release, Trump hit back on his Truth Social platform, calling Smith "deranged", and adding that he "was unable to successfully prosecute the Political Opponent of his 'boss'."

"To show you how desperate Deranged Jack Smith is, he released his Fake findings at 1:00 A.M. in the morning," Trump added in another post.

Judge Aileen Cannon, a Trump appointee, dismissed a separate case against the former and future president last year -- over Trump's handling of top secret documents after leaving the White House -- but charges are still pending against two of his former co-defendants.

Smith left the justice department last week, days after submitting his final report as special counsel.