"I'm here in New Hampshire for one simple reason: America cannot survive another four years of Barack Obama and Mitt Romney's the man to lead America and we need him now," Christie said, standing alongside Romney.
The endorsement was a surprise, coming just hours before a GOP presidential debate and just a week after the pugnacious, budget-cutting Christie disappointed party elders and top GOP donors when he decided last week that he wouldn't run for president in 2012.
In the intervening days, Romney and his chief rival, Texas Gov. Rick Perry, have been scrambling to win support from the donors and party elders who had been sitting on the sidelines and waiting for Christie to decide.
It's unclear just how much impact -- if any -- the endorsement will have with fewer than three months before the primary voting season begins and as Romney tries to position himself as the party's inevitable nominee.
Voters tend to make up their own minds about who to support.
But Christie's endorsement could influence groups Romney has struggled to win over -- a core segment of the GOP establishment that isn't enthused by him and the tea party, many of whom view him as insincere on issues they hold dear.
In a hastily arranged news conference on the debate sidelines, Christie said that the former Massachusetts governor could beat Obama and has the right mix of private sector and government experience to be president.
Christie also addressed what's perhaps Romney's biggest vulnerability -- the Massachusetts health care measure that he signed into law and that was a model for Obama's nationwide measure that conservatives detest.
The New Jersey governor said it was "completely intellectually dishonest" to link Romney's measure with Obama's. And Christie added of Romney: "I'm proud of him for doing what he thought was right" on health care in Massachusetts.
Romney, in turn, called Christie a "hero" because of his record of cutting government spending as governor of New Jersey.
Christie has closer ties to the former Massachusetts governor than to other candidates. Romney endorsed Christie when the former U.S. attorney ran for governor in 2009. And in January he became the first Republican presidential contender to visit Christie at the governor's mansion in Princeton.
Christie's financial supporters had been waiting for him to decide before backing a different candidate. The New Jersey governor's endorsement is a blow to Perry, as it's likely to send much of that cash to Romney. Several top Christie donors, including Home Depot financier Ken Langone, had already announced they would back Romney.
Romney already had an $18 million financial head start over Perry, who couldn't start raising money until August, when he announced his presidential bid. Perry raised more than $17 million in his first six weeks campaigning, showing he can keep pace with Romney's financial resources. Romney is expected to announce he raised more than $14 million between July and September.
Christie's backing could help solidify Romney's standing as the most logical candidate in a field that just became settled last week, after Christie and former Alaska Gov. Sarah Palin both decided not to run.
Romney also stands to benefit from the budget-cutting Christie's ties to the tea party, a group of voters that Romney has struggled to win over.
The endorsement comes as the race accelerates because of a compressed primary calendar. Because Florida moved its primary to the end of January, voting in Iowa and New Hampshire is likely to start just after the new year or even as early as December.
That's left Romney in a stronger position than the rivals who are still struggling to introduce themselves to voters. Romney has led most early national polls, and has a campaign structure that he's been building since he lost his 2008 bid for the nomination.
And so far, the GOP race has been a struggle to find an alternative to Romney. Perry and Minnesota Rep. Michele Bachmann have both tried to pitch themselves in that role, and now former Godfather's Pizza CEO Herman Cain has risen in national polls.
Perry and Bachmann have stumbled in their attempts to attack Romney: Perry flubbed a practiced criticism during his last debate; Bachmann struggled to hold onto her fast rise in popularity and has struggled to gain traction for her message that casts Romney as a moderate who can't be trusted. And Cain has yet to face the scrutiny that's focused on top-tier candidates.
Romney, meanwhile, has turned in one steady debate performance after another. He's making fewer mistakes on the campaign trail and he's been able to spend most of his time focused on attacking Obama.
He faced another test in Tuesday's debate. It's focused strictly on the economy, a subject that will give Romney, a former venture capitalist, the opportunity to focus on his campaign's core message.
Associated Press writers Kasie Hunt in Washington and Beth DeFalco in Trenton, N.J., contributed to this report.