A U.S. judge in Florida on Monday dismissed the criminal case accusing Donald Trump of illegally holding onto classified documents after leaving office, handing the Republican former president another major legal victory as he seeks a return to the White House.

U.S. District Judge Aileen Cannon, who was appointed to the bench by Trump, ruled that Special Counsel Jack Smith, who is leading the prosecution, was unlawfully appointed to his role and did not have the authority to bring the case.

It marked another blockbuster legal triumph for Trump, following a July 1 U.S. Supreme Court ruling that as a former president he has immunity from prosecution for many of his actions in office.

Cannon's ruling came two days after Trump was the target of an assassination attempt at a campaign rally in western Pennsylvania. Trump is set to be formally named the Republican presidential nominee in Milwaukee this week challenging Democratic President Joe Biden in the Nov. 5 U.S. election.

Prosecutors are likely to appeal the ruling. Courts in other cases have repeatedly upheld the ability of the U.S. Justice Department to appoint special counsels to handle certain politically sensitive investigations.

A spokesperson for Smith did not immediately respond to a request for comment.

At the very least, Cannon's ruling throws the future of the case into doubt. Smith is also prosecuting Trump in federal court in Washington on charges involving the former president's attempts to overturn the 2020 election, but his lawyers have not made a similar challenge to the special counsel in that case.

In the documents case, Trump was indicted on charges that he willfully retained sensitive national security documents at his Mar-a-Lago estate after leaving office in 2021 and obstructed government efforts to retrieve the material.

Two others, Trump personal aide Walt Nauta and Mar-a-Lago property manager Carlos De Olivera, were also charged with obstructing the investigation.

Trump's lawyers challenged the legal authority for Attorney General Merrick Garland's 2022 decision to appoint Smith to lead investigations into Trump. They argued that the appointment violated the U.S. Constitution because Smith's office was not created by Congress and the special counsel was not confirmed by the Senate.

Lawyers in Smith's office disputed Trump's claims, arguing that there was a well-settled practice of using special counsels to manage politically sensitive investigations.

"This ruling flies in the face of about 20 years of institutional precedent, conflicts with rulings issued in both the Mueller investigation and in D.C. with respect to Jack Smith himself," said Bradley Moss, an attorney who specializes in national security.

Moss also said the ruling raises the question of whether Smith will seek to have Cannon removed from the case.

Cannon's ruling is the latest and most consequential in a series of decisions she has made favoring Trump and expressing skepticism about the conduct of prosecutors. The judge previously delayed a trial indefinitely while considering a flurry of Trump’s legal challenges.

In an unusual move, she allowed three outside lawyers, including two who sided with Trump, to argue during a court hearing focused on Trump's challenge to Smith's appointment.

Conservative Supreme Court Justice Clarence Thomas also provided a boost to Trump's challenge to the special counsel. In an opinion agreeing with the court's decision to grant Trump broad immunity in the election-related case, Thomas questioned whether Smith's appointment was lawful using similar arguments to those made by Trump's lawyers.

Garland appointed Smith, a public corruption and international war crimes prosecutor, to give investigations into Trump a degree of independence from the Justice Department under Biden's administration.