Trump faces biggest legal threat as documents case opens in court

Donald Trump is due in court Tuesday to face dozens of felony counts of mishandling US government secrets, in the most serious yet of a firestorm of criminal probes threatening to derail his bid to win back the White House.

The former president plans to travel by motorcade for the 25-minute journey from his golf course in Miami to the federal courthouse, where he is expected to deny 37 counts of unlawfully retaining classified documents and obstructing efforts to get them back.

The Republican leader is running for reelection, and devoted supporters had already begun hitting the streets on the eve of the hearing -- with Miami police bracing for protests of up to 50,000 and prepared for the possibility of violence.

"There's never been anything like it. A witch hunt like this has never taken place," Trump told a local conservative Hispanic radio station after arriving in Miami from his summer home in New Jersey on Monday.

"When you look at what they've done, and when you look at the criminal acts and the horrible acts that they've committed, and then they come after me."

The pugnacious billionaire, who turns 77 on Wednesday, is accused of willfully hoarding dozens of clearly-marked government secrets he took unlawfully to his beachfront mansion in Florida when he left office in 2021, refusing to return them and conspiring to obstruct investigators seeking to recover them.

He is also charged with sharing sensitive US secrets with people who had no security clearance, in a much more serious case than any he has previously faced, with charges that can carry decades-long prison sentences.

The runaway frontrunner in the 2024 Republican primary has vowed to stay in the race regardless of the outcome of the case -- touching off a White House campaign that, for the first time in US history, pits a legal battle against an electoral one.

The 49-page indictment, dismissed by Trump as "ridiculous," included photographs showing boxes that were supposed to be in the National Archives stacked at Mar-a-Lago, his Palm Beach residence, in a ballroom and in a bathroom and shower.

Security was being ramped-up around Miami's Wilkie D Ferguson Jr Courthouse, with several protests planned including by a local chapter of the far-right Proud Boys group.

"We hope that tomorrow will be peaceful. We encourage people to be peaceful in demonstrating how they feel," Miami's Republican mayor Francis Suarez told reporters on Monday.

Trump is expected to fly to his golf club in Bedminster, New Jersey, afterwards to restate his innocence in a speech before supporters.

The Republican former president's legal woes are only just beginning, as he faces multiple felony counts in a financial fraud case in New York set to go to trial next March.

Jack Smith, the special counsel leading the documents probe, is also looking into Trump's involvement in the 2021 US Capitol riot, and state and federal investigators are scrutinizing his efforts to subvert the 2020 election.

Trump's allies in Congress and rivals for the presidential nomination have largely circled the wagons following his latest indictment, decrying the "weaponisation" of the government against conservatives.

Some Republican lawmakers have been criticized for rhetoric that could inspire violence, including Louisiana's Clay Higgins, who told supporters to "buckle up" and Arizona's Andy Biggs, who tweeted: "We have now reached a war phase. An eye for an eye."

The Southern District of Florida is known as a "rocket docket" court, legal slang for locations that push for swift justice, and authorities have not ruled out completing a trial before the 2024 election.

Much of the focus in the preliminary proceedings will be on District Court Judge Aileen Cannon, a Trump appointee who was allocated the case at random and will have enormous sway over how fast things move.

Cannon issued a series of rulings favorable to Trump earlier in the case that effectively jammed up the investigation for weeks until a conservative appeals court ruled she had acted beyond her authority.

Another judge will oversee the arraignment hearing itself.