Former special counsel Jack Smith says President-elect Donald Trump engaged in an “unprecedented” effort to overturn the voters’ will in 2020 and would have been convicted at trial if he had not won in November.

Mr. Smith outlined his conclusion in a 174-page report, which the Department of Justice sent to Congress overnight Monday.

The report says there is “substantial evidence” that Mr. Trump “engaged in an unprecedented criminal effort to overturn the legitimate results of the election in order to retain power.”

The lengthy document serves as Mr. Smith’s final stamp on the long-running probe that enraged Mr. Trump but faced lengthy delays and sputtered out as the Republican president-in-waiting turned his legal woes into fuel for his 2024 campaign.

The new report details Mr. Smith’s effort to investigate and prosecute Mr. Trump and those in his orbit over their coordinated efforts to overturn the 2020 results through alleged pressure campaigns and fake electors in the states.

Separately, Mr. Smith prosecuted Mr. Trump over his decision to take classified documents to his residence in Florida. The final report on that probe is being withheld while a legal appeal against Mr. Trump’s co-defendants in the documents case is pending.

Both cases against Mr. Trump were dismissed after he won the presidency, though Mr. Smith insisted that was not an exoneration.

“The office concluded that Mr. Trump’s conduct violated several federal criminal statutes and that the admissible evidence would be sufficient to obtain and sustain a conviction,” said Mr. Smith, who resigned this month as special counsel. “The department’s view that the Constitution prohibits the continued indictment and prosecution of a president is categorical and does not tum 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.”

Mr. Trump reacted furiously to the overnight release of the report, one week before he will celebrate his return to the White House.

“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!!!” Mr. Trump posted on Truth Social.

By contrast, former House Speaker Nancy Pelosi praised Mr. Smith as “courageous.” 

“The special counsel’s report outlines with painstaking detail that the actions knowingly taken by Donald Trump resulted in a sinister plot to subvert the Congress, shred the Constitution and halt the peaceful transfer of power,” said Mrs. Pelosi, California Democrat. “The facts remain clear — and our democracy demands that the American people know the important conclusions found in this report.”

Mr. Smith secured an indictment against Mr. Trump in a federal court in Washington in 2023 after two years of debate about Mr. Trump’s conduct following his 2020 election loss to President Biden.

Mr. Trump was impeached but not convicted by the Senate, and House Democrats used a select committee to investigate Mr. Trump’s post-election actions and the Jan. 6, 2021, riot at the U.S. Capitol.

Mr. Smith defended his office against complaints, mainly from the political left, that the Justice Department dragged its feet on the probe. Mr. Trump and his GOP allies said the probe was timed to interfere in the 2024 election.

“The office’s exceptional working pace ensured that its investigative work could be completed, charging decisions could be made, and any necessary indictments could be returned by the summer of 2023, long before the election,” Mr. Smith wrote. “The office had no interest in affecting the presidential election, and it complied fully with the letter and spirit of the Department’s policy regarding election year sensitivities.”

Mr. Trump is returning to the White House without the cloud of legal woes hanging over his head.

He was sentenced last week to no jail time or probation for his New York state felony conviction for falsifying business records. The judge said an “unconditional discharge” was the best way to resolve the case, given his status as president-elect.

A Georgia case is in limbo because Fulton County District Attorney Fani Willis was removed from it because of her romantic relationship with an investigation that posed a potential conflict.

U.S. District Judge Aileen Cannon in Florida dismissed the documents case, reasoning that Mr. Smith was unlawfully appointed as special counsel because the Senate did not confirm him and he was a private citizen when tasked with the case.

Mr. Smith contested that ruling but later moved to drop his appeal as it pertained to Mr. Trump, citing the DOJ’s Office of Legal Counsel’s position against prosecuting sitting presidents.

But he left the appeal intact for two co-defendants, Waltine Nauta and Carlos de Oliveira.

“Unlike defendant Trump, no principle of temporary immunity applies to them,” assistant special counsel James I. Pearce wrote in court papers.

The lingering appeal against Mr. Nauta and Mr. Oliveira sparked a fight over the release of Mr. Smith’s report of the documents probe. It is under wraps for now, but the Justice Department wants to share it with House and Senate committees.