New Jersey Gov. Chris Christie issued a surprise endorsement of Mitt Romney for president yesterday — coming a week after the pugnacious Garden State heavyweight ruled out his own run for the White House.

And Christie went all in for Romney, appearing with the former Massachusetts governor in New Hampshire just hours before last night’s Republican debate.

Christie said he backed Romney because he’s the class of the field and has the best chance to defeat President Obama.

He said Romney’s mix of executive experience as a businessman and a governor is unmatched by the other Republican candidates.

“We have seen what it’s like to elect a legislator with no executive experience to the White House,’’ Christie said during a press call with reporters, referring to Obama.

Romney jubilant backers said that Christie’s endorsement was a big catch for their team.

“This is huge. This is as good as it gets,” said Staten Island Republican power broker Guy Molinari, a top New York Romney backer.