The only thing surprising about this is the timing:

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., the site of the Republican presidential debate being held Tuesday night. The endorsement will be made in advance of the debate.

Marking a fast turnaround for Romney, the 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.

Well, who else would Christie endorse in this race?  Politically he and Romney are quite similar — northeastern Republican governors in substantially blue states, with questionable adherence to conservative orthodoxies.  Had Christie gotten into the race, he would have competed for the moderates that Romney attracts now, not  conservatives who might prefer Herman Cain or even Rick Perry.  Christie’s endorsement was inevitable.

But why the rush?  Just a  few days ago, Christie seemed unconvinced enough to be willing to jump into the race himself — even though Romney’s been running since, oh, 2008 or so.  Maybe the process of considering a run just super-convinced Christie that Romney was the only candidate for this cycle.  Christie could also have kept himself relevant in the process by holding off on an endorsement until just before the primaries, and that would also have given the Romney campaign one last boost just as Iowans went to the caucuses or when Santa started dropping down chimneys, whichever comes first.

The obvious conclusion is that Romney’s worried about an inability to move upward and the continued rise and fall of the Not-Romneys in the race.  Be sure to read Allahpundit’s observation last night on the GOP’s track record in presidential elections when no one candidate has emerged by this stage for an explanation for why Romney might want to put this one away now.  By getting ahead of the news cycle with a big endorsement just before tonight’s debate, it could also relieve pressure on Romney to make news by attacking Herman Cain — or possibly hint that Romney wants a knockout punch badly enough to risk engaging the conservative base’s favorite of the moment.