The surprise endorsement comes as the Trump campaign continues its muscular charge toward the Republican nomination, bolstered by double-digit leads in national polls. A strong showing in the March 1 “Super Tuesday” primaries, when 11 states will cast ballots in the GOP race, could dramatically expand Trump's delegate lead.
Trump's critics have persistently scrutinized his grasp of public policy and his bombastic style. The New Jersey governor’s endorsement potentially recasts the GOP nominating contest, giving Trump — already the undisputed front-runner — the weight and validation of a respected Republican executive officeholder.
Christie himself raised questions about Trump's qualifications during his own failed White House bid, which he suspended earlier this month after a weak showing in the New Hampshire primary. But on Friday Christie praised the real estate mogul and his campaign.
“Donald Trump is someone who, when he makes a promise, he keeps it,” Christie said. "No one is going to get inside this guy's head. There is no better fighter than Donald Trump."
Christie added that he believes the party needs to elect the candidate most likely to beat Democratic front-runner Hillary Clinton. Trump, the governor said, is “undoubtedly” the best person for the job and “is rewriting the playbook of American politics.”
Both Christie and Trump emphasized that they have known each other for several years and have become friends. Christie said that he finalized his decision to publicly support Trump after ending his own presidential campaign and discussing the matter with his family. "It was clear the only choice was Donald Trump, the best choice was Donald Trump," he said.
They finalized the endorsement Thursday, the two told reporters.
The surprise backing is a blow to Sen. Marco Rubio (R-Fla.), who is seeking to become the main challenger to Trump for the Republican nomination. On Friday, Christie blasted Rubio's escalating personal attacks against the billionaire, calling them an act of desperation. Christie added that Rubio represents the Washington establishment.
“I find it fascinating that somebody who barely shows up for work … is going to talk about somebody being unprepared,” Christie said.
“He’s a desperate guy,” Trump added later. “He is not presidential material. He doesn’t have the demeanor. He is a nervous nellie.”
Sen. Ted Cruz (R-Tex.), who has been locked in a bitter fight with Rubio over second place, characterized the endorsement as a blow to Rubio's chances of uniting establishment Republicans behind his candidacy. Both Cruz and Rubio are seeking to emerge as Trump's chief rival and to consolidate anti-Trump votes before the billionaire can amass a prohibitive delegate lead.
"I think the endorsement was no doubt troubling news for the Rubio campaign. I like Chris. I don’t think the endorsement was a big surprise to many observers," Cruz told reporters in Nashville. "Our focus is real simple: uniting conservatives on Super Tuesday. I don’t think this endorsement has any significant impact on that," he said."
Trump dismissed a question from reporters about whether Christie was being considered for the vice-presidential nomination or for a Cabinet position, saying that they never discussed that when Christie called to offer his endorsement. The governor said he is committed to fulfilling his term in office, which ends in 2018.
Trump also strongly dismissed rumors that he was once considering Rubio as his running mate, saying he never thought to choose the Florida senator for that role.
Speaking to thousands of Trump supporters later Friday afternoon, Christie again knocked Rubio: “President of the United States is not a no-show job like you treated the United States Senate.”
Christie was not Trump's only gubernatorial endorsement Friday: Maine Gov. Paul LePage (R) also endorsed the real estate mogul during a radio interview on the "Howie Carr Show." "I think he could be one of the greatest presidents ever," LePage said.
Katie Zezima contributed to this story.