Support for Christie among Republican primary voters is in the low single digits -- very low. His favorables are underwater, and it's not as though they didn't know him. A significant segment of Republican donors is legally unavailable to the governor of NJ. His claim used to be that he was electable, and he is not.
So, why is he running?
Well, he's an ex-prosecutor. I think he's considering the optics of a federal indictment of a presidential candidate of the opposing party. He'd claim that he was being prosecuted for the political motive of destroying a candidacy which threatened the Democrats' chances. He could persuade most Republicans of that, and even in New Jersey, a jury would include a fair number of Republicans. All an acquittal requires is for one of those to remain persuaded.