IOWA, United States (AP) — Mike Pence opened his presidential bid with an unusually forceful critique of former President Donald Trump over January 6, his temperament and abortion on Wednesday as he became the first vice-president in modern history to challenge his former running mate.

Pence, according to excerpts released by his campaign, will tell an audience in the Des Moines suburbs that Trump betrayed his voters' trust after the 2020 election when he tried to convince his supporters that Pence had the power to keep them in office.

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

Pence will criticise leaders who can't "distinguish between starting fights and finishing them".

"Most Americans treat each other with kindness and respect — even when we disagree," he will say. "It's not too much to ask our leaders to do the same."

Pence, who supports a national ban on abortion, will also criticise Trump's current rhetoric on the topic, even after Trump appointed the Supreme Court justices who overturned Roe v Wade. Trump has declined to say what limits he supports nationally and has blamed some midterm candidates' extreme rhetoric for their losses last November.

"After leading the most pro-life administration in American history, Donald Trump and others in this race are retreating from the cause of the unborn," Pence will say. "The sanctity of life has been our party's calling for half a century — long before Donald Trump was ever a part of it. Now he treats it as an inconvenience, even blaming election losses on overturning Roe v Wade."

With Pence's entry into the race, on his 64th birthday, the GOP field is largely set. It includes Trump, who's leading in early polls, Florida Gov Ron DeSantis, who remains in second, former United Nations Ambassador Nikki Haley, South Carolina Senator Tim Scott, former New Jersey Gov Chris Christie, former Arkansas Gov Asa Hutchinson and North Dakota Gov Doug Burgum, who also launched his campaign Wednesday.

Pence is staking his presidential hopes on Iowa. His campaign will test the party's appetite for a socially conservative, mild-mannered and deeply religious candidate who has denounced the populist tide that has swept through his party under Trump. And it will show whether Pence still has a political future after January 6, 2021, when a large portion of GOP voters still believe Trump's lies that the 2020 election was stolen and that Pence had the power to reject the results of the election, won by Democrat Joe Biden.

Pence and his advisers see Iowa — the state that will cast the first votes of the GOP nominating calendar — as key to his potential pathway to the nomination. Its caucusgoers include a large portion of evangelical Christian voters, whom they see as a natural constituency for Pence, a social conservative who supports a national ban on abortion and often talks about his faith. They also think Pence, who represented Indiana in Congress and as governor, is a good personality fit with the Midwestern state.

"We believe the path to victory runs through Iowa and all of its 99 counties," said Scott Reed, co-chair of a super PAC that launched last month to support Pence's candidacy.

But Pence also faces steep challenges. Despite being one of the best-known Republican candidates in the crowded field, he is also saddled with high unfavorability ratings. Trump critics consider him complicit in the former president's most indefensible actions, while Trump loyalists have maligned him as a traitor.

A CNN poll conducted last month found 45 per cent of Republicans and Republican-leaning independents said they would not support Pence under any circumstance. Only 16 per cent said the same about Trump.

Pence's favourability has also slipped in Iowa, according to The Des Moines Register/Mediacom Iowa Poll. The poll also found Pence with higher unfavourable ratings than all of the other candidates it asked about, including Trump and DeSantis, with 26 per cent of Republicans polled saying they have a "somewhat" or "very" unfavourable view of him.

But Pence, who has visited Iowa more than a dozen times since leaving office, has been warmly welcomed by voters during his trips. During a "Roast and Ride" event over the weekend that drew a long list of 2024 candidates, Pence stood out as the only contender to actually mount a Harley and participate in the event's annual motorcycle ride. When he arrived at a barbecue at the state fairgrounds, he moved easily from table to table, greeting and chatting with attendees.

But there remains lingering scepticism among many Republican voters who still believe Pence could have stopped Biden from becoming president. Trump's lies about mass voting fraud and Pence lacking the "courage" to do the right thing led a mob of his supporters to violently storm the Capitol, with some chanting "Hang Mike Pence!"

Pence advisers say they recognise the challenge and intend to explain to voters directly that Pence was adhering to his constitutional duty and never had the power to impact the vote.

"I think it's something you have to walk straight through," said his long-time adviser Marc Short.

Beyond January 6, his team sees their primary goal as reintroducing Pence to a country that largely knows him as Trump's second-in-command. They want to remind voters of his time in congressional leadership and as governor and envision a campaign heavy with town halls, house parties and visits to local diners and Pizza Ranch restaurants — more intimate settings that will help voters get to know him personally.