Former US Vice President Mike Pence has unleashed on his former boss, Donald Trump, as he declared his candidacy for the 2024 Republican race for the White House.

Mr Pence’s campaign filed a declaration of candidacy with the Federal Election Commission on Monday (local time), and he launched his campaign with a speech in Iowa on Wednesday.

It marked a turning point in Mr Pence’s relationship with Mr Trump, as he argued the former president’s attempts to overturn the 2020 election – and his request for Mr Pence to violate the Constitution in the process – made him unfit for office.

“The American people deserve to know on that fateful day, President Trump also demanded I choose between him and our Constitution,” Mr Pence said.

“Now, voters will be faced with the same choice. I chose the Constitution, and I always will.”

“The American people must know that leaders in the Republican Party will keep our oath to support and defend the Constitution, even when it is not in our political interests.

“One last word that in part brings us here today: I believe that anyone who puts themselves over the Constitution should never be president of the United States, and anyone who asks someone else to put them over the Constitution should never be president of the United States again.”

A staunch social conservative and a former governor of Indiana, Mr Pence initially stood loyally by Mr Trump. But in recent months he has increasingly distanced himself from the embattled former president, saying Mr Trump’s encouragement of the rioters who attacked the US Capitol on January 6, 2021, endangered Mr Pence and his family, who were in the building at the time.

Mr Pence joins a growing field of Republican candidates, which includes Mr Trump, US Senator Tim Scott and former South Carolina Governor Nikki Haley.

North Dakota Governor Doug Burgum is also reportedly planning to enter the race on Wednesday.

Former New Jersey Governor Chris Christie jumped in on Tuesday, bringing the total number of Republican candidates into double digits.

That number concerns many Trump opponents inside the Republican Party. They fear the anti-Trump vote could be split and hand the party’s nomination to the former president.

Trump a target in documents probe

Mr Pence’s announcement came as federal prosecutors notified Mr Trump he was the target of an investigation into his handling of classified materials, adding to his legal troubles as he campaigns for the White House in 2024.

The Justice Department typically notifies people when they become targets of an investigation to give them an opportunity to present their own evidence before a grand jury.

The notification does not necessarily mean Mr Trump will be charged.

Mr Trump’s legal team was notified on Monday, a person familiar with the matter said on Wednesday.

Although there are some signs the documents investigation is drawing to a close, the timing of when a person was told they were a target could not necessarily be used as a predictor of when charges might be brought, said David Schoen, a lawyer who represented Trump ally Steve Bannon during his criminal trial on contempt of Congress charges.

“Sometimes they are issued at the beginning of a long investigation and sometimes at the conclusion of an investigation,” he said.

Mr Trump has repeatedly described the multiple investigations as politically motivated.

A federal grand jury has been investigating Mr Trump’s retention of classified materials after leaving the White House in 2021.

A second criminal investigation is looking into alleged efforts by Mr Trump and his allies to overturn his 2020 election loss to Democrat Joe Biden.

A representative for Special Counsel Jack Smith, who is leading the probes, declined to comment.

Investigators in August 2022 seized roughly 13,000 documents from Mr Trump’s Mar-a-Lago estate in Palm Beach, Florida.

Of those, 100 were marked as classified, even though one of Mr Trump’s lawyers had previously said all records with classified markings had been returned.

Mr Trump has defended his retention of documents, suggesting he declassified them while he was president.

Mr Trump has not provided evidence of this and his lawyers have not made that argument in court filings.

Mr Trump is the first current or former US president to face criminal charges, having pleaded not guilty in April to felony charges for allegedly falsifying business records relating to hush money paid to a porn star before the 2016 presidential race.

His legal woes are growing.

A jury in federal court in Manhattan in May decided in a civil lawsuit that Mr Trump must pay $US5 million ($7.5 million) in damages for sexually abusing former Elle magazine columnist E Jean Carroll and then defaming her by branding her a liar.

Mr Trump also faces a criminal investigation by a county prosecutor in Georgia relating to his efforts to undo his 2020 election loss in that state.

-with agencies