Former US vice president Mike Pence launched his presidential campaign Wednesday by framing the Republican nomination as a choice between "reckless" Donald Trump and the Constitution -- arguing that his old boss's bid to overturn the last election should bar him from returning in 2024.

Offering a spirited defense of the Trump White House's policies, the deeply religious former radio talk show host and Indiana governor said he was proud to stand with his running mate "every single day" during the 2017-21 administration.

But he drew the line at the then-president's incitement of a crowd to storm the Capitol on January 6, 2021, as Pence was inside, overseeing the certification of Joe Biden's election win.

"As I've said many times, on that fateful day, president Trump's words were reckless and endangered my family and everyone at the Capitol," Pence told supporters in Ankeny, Iowa.

"The American people deserve to know that on that day, President Trump also demanded that I choose between him and the Constitution. Now voters will be faced with the same choice. I chose the Constitution and I always will."

Pence honed a reputation as an unstintingly loyal vice president who stuck with Trump throughout a scandal-plagued four years in the White House, and brought the religious right into the tent.

But the evangelical Christian's refusal to follow Trump's urging and use his role as president of the Senate to sabotage the 2020 election made him a pariah with Trump's fanatical base -- and the populist firebrand himself.

Pence was forced to flee for his life when a mob directed by Trump to march on the Capitol broke through barricades and called for the vice president's execution.

- 'Decency and civility' -

Pence, who in a launch video earlier Wednesday insisted that "God is not done with America yet," is the first modern vice president to challenge his old running mate for his party's nomination.

His announcement underscored the tightrope that he will have to walk on the campaign trail as he attempts to distance himself from the chaos of the Trump years while taking credit for the gains he believes the country made.

Pence attacked Trump for backing off conservative policies such as tough abortion curbs and fiscal responsibility, and accused him of breaking a promise "on day one" to govern with "decency and civility."

When asked about media reports that Trump's lawyers had been informed their client was the target of an investigation into the mishandling of classified documents after he left the White House, Pence told a CNN town hall audience that "no one's above the law."

"I would just hope there would be a way for them to move forward without the dramatic and drastic and divisive step of indicting a former president of the United States," he added.

Pence, who was celebrating his 64th birthday, announced his presidential run a day after former New Jersey governor Chris Christie joined the contest, promising to be the only candidate who would not pull his punches against Trump -- still the dominant Republican figure for much of the country.

- 'Small, pathetic man' -

Florida governor Ron DeSantis and two former governors, Nikki Haley and Asa Hutchinson, are also in the race.

Polls show Trump as the overwhelming early frontrunner, regularly posting leads on second-placed DeSantis in excess of 30 points. None of the other candidates -- Pence included -- is achieving double figures.

DeSantis traveled to southern Arizona Wednesday, where he touted his tough stance on immigration and defended his state's decision to send dozens of mainly Venezuelan migrants to California on charter flights from Texas in recent days.

California's Democratic governor Gavin Newsom threatened DeSantis with kidnapping charges -- calling him a "small, pathetic man" -- over the taxpayer-funded operation, after officials said the migrants had been misled into boarding the planes with false promises of jobs.

DeSantis responded by criticizing "sanctuary" cities and states, like California, and called for the border to be "shut down" at a round-table discussion in Sierra Vista with law enforcement officials from Florida, Arizona and Texas.

"That's the policies that they've (staked) out," DeSantis said, criticizing California's more relaxed approach to immigration control.

"And then what? When they have to deal with some of the fruits of that, they all of a sudden become very, very upset about that."

Target of classified docs probe

US prosecutors have told Donald Trump's lawyers that he is the target of a probe into his handling of classified documents after leaving the presidency, in a sign he might be indicted, US media reported Wednesday night.

The notice came from the office of special counsel Jack Smith, The New York Times, CNN and other new outlets said, and gives the clearest sign yet that prosecutors are moving closer to charging the former president, who is seeking a return to the White House in 2024.

The Times, citing "two people familiar with the matter," said it was not clear when Trump's team had been told he was subject in the investigation.

Special counsel Smith, named by Attorney General Merrick Garland, has been looking into a cache of classified documents that Trump had stored at his Mar-a-Lago residence after leaving the White House.

The FBI carted away some 11,000 papers after serving a search warrant on Mar-a-Lago in August, and Trump could face obstruction of justice charges after spending months resisting efforts to recover the trove.

Trump has repeatedly denied any wrongdoing.

"No one has told me I'm being indicted, and I shouldn't be because I've done NOTHING wrong," Trump posted on his Truth Social network Wednesday.

"I have assumed for years that I am a Target of the WEAPONIZED DOJ & FBI," Trump, who has already been indicted in a separate case, added.

Investigators have been scrutinizing several suspected attempts to obstruct them from getting access to documents and footage from a security camera near the storeroom at Mar-a-Lago where documents were kept.

Trump has openly acknowledged taking and storing the documents, undermining his lawyers' suggestion that he took the stash inadvertently in the confusion of a chaotic departure.

Meanwhile, investigators have obtained an audio recording of Trump in which he acknowledges he held onto a classified Pentagon document after he left the White House, according to US media.

And on Monday, US media also reported that Trump's lawyers John Rowley, James Trusty and Lindsey Halligan met with Justice Department attorneys -- they were seen by reporters arriving at the DoJ building in Washington, but did not make any comment.

The Washington Post reported at the time that Trump's lawyers had come to make their case that their client should not face any charges.

- 'Dramatic... and divisive' -

The former president has already been charged with dozens of financial crimes as part of an alleged hush money scheme to silence a porn star claiming she had sex with him, and is due to go on trial next March, in the middle of primary election season.

Trump is facing a growing list of Republican primary opponents, including his own former vice president Mike Pence, who avoided taking an absolute position when asked about the probe during a CNN town hall interview Wednesday night.

"Let me be clear that no one's above the law," Pence said.

"I would just hope there would be a way for them to move forward without the dramatic and drastic and divisive step of indicting a former president of the United States," he added.

Fellow 2024 candidate former Arkansas governor Asa Hutchinson took a stronger tack, urging Trump to drop out of the presidential race.

"Donald Trump is the target of an ongoing criminal investigation and he should step aside & put the good of the country above his candidacy," he tweeted Wednesday night.

In addition to the hush money and documents probes, Trump is also being investigated over his efforts to overturn his 2020 election loss in the southern state of Georgia and his involvement in the January 2021 storming of the US Capitol by his supporters.

Other politicians have been found to have been in possession of government documents at their private homes in recent months.

In January, Garland named a separate special counsel to examine whether President Joe Biden violated any law after the discovery of some classified documents at his former office and in the garage of his home in Wilmington, Delaware.

And Pence was recently cleared of any wrongdoing over materials he uncovered at his home in Indiana.