Former US vice president Mike Pence has launched his presidential campaign 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 defence of the Trump White House's policies, the deeply religious former radio talk show host and Indiana governor said on Wednesday 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 base — and the populist firebrand himself.

Pence was forced to flee for his life when a pro-Trump mob marched on the Capitol, broke through barricades and called for the vice president's execution.

'God is not done with America yet'

Pence, who in a launch video earlier on 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."

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.

Other runners

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 front-runner, regularly posting leads on second-placed DeSantis in excess of 30 points.

None of the other candidates — Pence included — is achieving double figures