New Jersey Gov. Chris Christie, just one week after definitively announcing he will not run for president in the 2012 race, plans to endorse Mitt Romney for the job Tuesday afternoon, Fox News has learned.

The event is set to take place in Hanover, N.H., just hours before the Republican candidates gather for a debate nearby at Dartmouth College. In securing the support of Christie, Romney will have at his side a tough-talking governor who during his two years in office has built a reputation as a fiscal hawk not shy about taking on the public employee unions.

The former Massachusetts governor is enlisting Christie at a time when his front-runner status is once again being challenged. Romney saw his lead slip after Texas Gov. Rick Perry jumped into the race in August. Perry has since fallen back in the polls, but businessman Herman Cain broke through to within a few points of Romney in several recent national polls.

Marking a fast turnaround for Romney, the new endorsement comes one week to the day since Christie called a press conference in Trenton to end once and for all the speculation that he would make a late entrance into the race.

Christie for months had said he would not run, but acknowledged that encouragement from others had him rethinking the decision in recent weeks. But he said he never changed his mind, and determined he had too much unfinished business to take care of in his home state.

The endorsement of Romney should leave no doubt that Christie is out as a potential candidate, but in as a potential high-profile surrogate for the former Massachusetts governor.

Romney also has secured the endorsement of former Minnesota Gov. Tim Pawlenty, who bowed out of the race in August.