Republican former vice president Mike Pence has launched his hotly-anticipated challenge to his one-time boss Donald Trump for the party's 2024 White House nomination, papers filed with the US Federal Election Commission showed Monday.

The 63-year-old evangelical Christian was scheduled to kick off his presidential campaign officially on Wednesday with a video in the early voting state of Iowa, joining an already crowded field.

Pence will then make his pitch to the nation at a live televised town hall event at 9:00 pm (0100 GMT Thursday), setting up the unusual scenario of two former running mates becoming rivals.

The former vice president spent his 2017-21 tenure as vice president by Trump's side, honing his reputation as an unwaveringly loyal deputy who brought the religious right into the tent and who was willing to defend the president against any accusation.

But he became a pariah in Trumpworld after rejecting the Republican leader's demands that he overturn the 2020 election in his role as president of the Senate.

Berated constantly by Trump after Joe Biden's victory -- and even heckled with chants of "traitor!" at a conservative conference in Florida -- Pence had continued to praise his assailant in public.

That eventually changed as Trump's torrent of false claims of election fraud led to a mob chanting for Pence to be hanged at the US Capitol.

Since the riot, Pence has called out Trump for endangering his family and other people at the Capitol, and has emphasized his differences with Trump on issues ranging from the handling of Russian leader Vladimir Putin to abortion rights.