Mitt Romney’s big endorsement: Chris Christie

Just one week after announcing he would not run for president, New Jersey Gov. Chris Christie is endorsing Mitt Romney for the Republican nomination, sources confirm.

Christie will attend Tuesday night’s Republican debate in New Hampshire as Romney’s guest. The Romney campaign had announced the former Massachusetts governor would address supporters this afternoon with a “special guest.”

When he reaffirmed his decision not to be a presidential candidate, Christie had suggested he was not likely to endorse one of the official candidates soon.

“If I feel like there’s someone in the field who gives us the best chance to defeat the president, I’ll endorse that person and I’ll work hard for that person. But I’m not in a position today to make that judgment,” he said at a news conference Oct. 4.

But the pairing of the two Northeastern Republicans seems a natural fit. And it gives Romney a major lift as the GOP field has settled; Christie was one of the most sought-after surrogates of the 2010 campaign, including a well-attended stop in Iowa.

Christie’s endorsement comes one day after Romney announced the support of two establishment Republicans -- former New Hampshire senator and governor Judd Gregg, and former Florida senator and Republican National Committee chairman Mel Martinez.

Christie was elected governor in 2009, after serving for nearly a decade as New Jersey’s U.S. attorney.

Maeve Reston contributed to this report.