Republican former New Jersey governor Chris Christie has jumped into the 2024 race for the White House.
Mr Christie filed his paperwork with the Federal Election Commission ahead of an evening announcement planned in Manchester, New Hampshire.
He presents a novel challenge as the only Republican contender so far willing to land genuinely damaging blows on former president Donald Trump.
The 60-year-old Newark native came sixth in New Hampshire seven years ago and eventually endorsed Mr Trump.
He served as a key advisor before the pair fell out over Mr Trump'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 criticised Mr Trump on all manner of issues, highlighting the escalating criminal probes targeting the embattled billionaire, trashing his false claims of election fraud and dubbing him "Putin's puppet" over his isolationist stance on Russia's invasion of Ukraine.
Mr Christie's launch comes a day after former vice president Mike Pence filed the paperwork for his own White House bid, setting up an unusual scenario in which two former running mates becoming rivals.
Mr Pence plans an official campaign launch tomorrow in the early voting state of Iowa.
He will join an already crowded field that includes Florida governor Ron DeSantis, who is widely seen as Mr Trump's closest competition.
Former governors Nikki Haley and Asa Hutchinson are also in the race, while North Dakota's chief executive Doug Burgam is due to announce tomorrow.
