“Donald Trump made us smaller by dividing us even further and pitting us one against the other,” Christie said, suggesting that the former president was part of a broader political effort to “paint all Republicans with just one brush.”

He added that President Joe Biden “is doing the same thing, just on the other side”.

Christie enters a growing primary field that already includes Trump, Florida Governor Ron DeSantis, former United Nations ambassador Nikki Haley and US Sen. Tim Scott of South Carolina. Former vice-president Mike Pence will be formally launching his own campaign in Iowa on Wednesday.

During his time as governor, Christie established a reputation as a fighter with a knack for creating viral moments of confrontation. But he faces an uphill battle to the nomination in a party that remains closely aligned with the former president, despite Trump’s re-election loss in 2020 and Republicans’ poorer-than-expected showing in the 2022 midterm elections.

Christie has become a fierce Trump critic in recent years and is now casting himself as the only Republican presidential candidate willing to openly pick fights with the former president – while also warning that the party failing to fully confront him will spark a repeat of the 2016 Republican primary, when Trump rolled over a host of alternatives with more political experience who split the support of voters opposing him.

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Anti-Trump Republicans are particularly eager to see Christie spar with Trump on a debate stage – if, of course, Trump agrees to participate in primary debates and Christie meets the stringent fundraising criteria set by the Republican National Committee for participation.

JP Marzullo, a former state representative and former vice-chairman of the New Hampshire Republican Party, previously backed Trump but is now supporting Christie.

“I think he’ll actually unite some of the voters, and he’ll get to independents,” Marzullo said of the former governor, adding: “I think it’s time for a change.”

Christie’s campaign will test the appetite among Republican voters for someone who has expressed support for many of Trump’s policies but has criticised the former president’s conduct. The former governor has rejected Trump’s lies that the 2020 election was stolen and has urged the party to move on or risk future losses.

Other Republicans with similar views, including former Maryland governor Larry Hogan and New Hampshire Governor Chris Sununu, have opted against their own campaigns, expressing concerns that having more candidates in the race will only benefit Trump.

Christie was at one point seen as one of the Republican Party’s brightest political stars as the popular Republican governor of a Democratic state. But despite persistent urging from top donors and party officials, he declined to run for president in 2012.

By the time he announced in 2016, his reputation had been tarnished by the “Bridgegate” scandal in which aides were accused of wreaking traffic havoc in Fort Lee, New Jersey, in an apparent effort to punish the city’s mayor for failing to endorse his re-election bid.

In the packed 2016 Republican primary, Christie portrayed himself as a brash, tough-talking East Coaster who could “tell it like it is” – only to be eclipsed by the brasher Trump.

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Christie opting to start his 2024 bid at a New Hampshire town hall recalled his first run at the White House, when he focused on the state, holding dozens of New Hampshire town hall events only to finish sixth in its primary. He dropped out of that race afterward.

Two weeks later, Christie stunned even some former aides when he endorsed Trump, becoming the first sitting governor and former rival to get behind the emerging Republican front-runner. His announcement undercut rival Marco Rubio at a crucial moment – the day after a debate that had been seen as a possible turning point in the race – helping to pave the way for Trump’s nomination and eventual win.

“The line of supporting Donald Trump starts behind me,” Christie has said.

The former governor, who has known Trump for nearly 20 years, has had a complicated friendship with the former developer and reality television star.

At times, he was one of Trump’s closest advisers: he was on the shortlist to serve as Trump’s vice-president, oversaw Trump’s early White House transition efforts, said he was offered – and turned down – multiple Cabinet positions, and helped Trump prepare for each of his general election debates in 2016 and 2020.

(It was during those debate preparations that Christie believes he caught Covid-19, landing him in intensive care.)

But Christie also clashed with Trump at times and has described the former president’s refusal to accept his 2020 election loss to Democrat Joe Biden as a breaking point.

In appearances and interviews, Christie says he was “incredibly disappointed and disillusioned” by Trump’s refusal to concede, which culminated in his followers’ violent storming of the Capitol on January 6 in an effort to halt the certification of Biden’s win.