Chris Christie, the former Republican governor of New Jersey, joined the 2024 campaign for the White House on Tuesday.
Mr Christie, 60, launched his campaign with the slogan "Because the truth matters" at an event in Manchester, New Hampshire, which holds the nation's first Republican primary after Iowa's lead-off caucuses.
“Donald Trump made us smaller by dividing us even further and pitting us one against the other,” he said.
Earlier, Mr Christie filed his paperwork with the Federal Election Commission to formally launch his bid, presenting a novel challenge as the only Republican contender so far willing to land genuinely damaging blows against Mr Trump.
The Newark native came sixth in New Hampshire seven years ago and eventually endorsed Mr Trump, serving as a key adviser before the pair fell out over the former president's refusal to accept his 2020 election defeat.
Mr Christie has already begun needling his former friend, arguing last month that the de facto Republican leader was “afraid” of debating serious opponents.
Mr Trump has indicated that he may skip at least one of the first two Republican primary debates, expressing a reluctance to share the limelight with lower-polling rivals.
“If he really cares about the country – and I have deep questions about that – but if he really cares about the country, then he's going to get up there and he shouldn't be afraid,” Mr Christie told conservative radio host Hugh Hewitt.
Mr Christie has assailed Mr Trump on all manner of issues, highlighting the criminal investigations into the embattled billionaire, scoffing at his false claims of election fraud and calling him "[Vladimir] Putin's puppet” over his isolationist stance on Russia's invasion of Ukraine.
The campaign launch comes a day after former vice president Mike Pence filed the paperwork for his White House bid, setting up an unusual scenario in which two former running mates become rivals.
The evangelical Christian plans to hold an official campaign launch on Wednesday, also in Iowa.
Polls show Mr Trump as the overwhelming early front-runner, regularly posting leads on Florida Governor Ron DeSantis in excess of 30 points.
No other candidate has reached double figures.
Agence France-Presse contributed to this report