Donald Trump would have been convicted for his "criminal efforts" to retain power after the 2020 election if the case had not been dropped because of his November White House victory, special counsel Jack Smith said Tuesday.

In a 137-page final report on the results of his historic prosecution of the former and future president, Smith laid out the evidence amassed against the 78-year-old Trump.

"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," he said.

Smith, who was appointed by Attorney General Merrick Garland, brought two federal cases against Trump -- for seeking to overturn the results of the election he lost to Joe Biden and mishandling top secret documents after leaving the White House.

Neither case came to trial and Smith dropped the charges in line with a Justice Department policy of not prosecuting a sitting president.

This new stinging rebuke from Smith means Trump will have something of a cloud hanging over him as he takes the oath of office Monday at the Capitol, ground zero of the insurrection he inspired after losing the 2020 election.

But it falls far short of the punishment which Trump critics had hoped for from at least one of four indictments against him.

In the end, after years of legal wrangling that Trump survived first by delaying tactics and ultimately by winning re-election, this is about all Trump critics can point to as justice -- the special prosecutor's assertion that, had Trump gone to trial in the election subversion case, he would have been found guilty.

Specifically, Trump was charged with conspiracy to defraud the United States and conspiracy to obstruct an official proceeding -- the session of Congress held to certify Biden's win that was violently attacked on 6 January 2021 by a mob of Trump supporters.

In his report, Smith accused Trump of using "fraud and deceit" to overturn the election results.

"When it became clear that Mr Trump had lost the election and that lawful means of challenging the election results had failed, he resorted to a series of criminal efforts to retain power," the special counsel said.

"This included attempts to induce state officials to ignore true vote counts (and) to manufacture fraudulent slates of presidential electors in seven states that he had lost," Smith said.

"Trump engaged in these efforts even though trusted state and party officials had told him from the outset that there was no evidence of fraud in the election."

Trump unsuccessfully pressured vice president Mike Pence not to certify the election results, Smith said, and on 6 January he directed "an angry mob to the United States Capitol."

Trump's untruths included dozens of demonstrably false claims that large numbers of ineligible voters, such as non-citizens, had cast ballots, and that voting machines had changed votes, the special counsel said.

'Laughable'

Trump lashed out at Smith following the release of the report, calling him "deranged" and claiming his prosecution was politically motivated.

"Jack is a lamebrain prosecutor who was unable to get his case tried before the Election, which I won in a landslide. THE VOTERS HAVE SPOKEN!!!" Trump said.

Smith flatly rejected the allegations of political bias in a letter to Garland that accompanied his final report.

"The claim from Mr Trump that my decisions as a prosecutor were influenced or directed by the Biden administration or other political actors is, in a word, laughable," he said.

Trump's lawyers tried but failed to block the report's release, calling it "undertaken in bad faith, and contrary to the public interest."

Smith has also prepared a report into Trump's mishandling of classified documents but it is being withheld because charges are pending against two of his former co-defendants.