Are the Dems really worried about a challenge from Christie?
There's a Daily Beast article about why the Dems gave Christie a pass that I'm not even going to bother linking to because it doesn't talk about anything substantial.
I don't know how disgruntled Christie was over the V.P. nomination going to Ryan or whether he and the Dems struck a deal that navigated Cory Booker, then seen as a tough challenger out of contention and into a seemingly easy Senate race and gave Obama a lot of positive publicity.
That kind of thing takes place on a whole other level.
But the main use that Democrats have gotten out of Christie is to use him as the "Good Republican". The Democrats always need their "Good Republican" to hold up as an example of how impossible it is to work with the rest of the Republican Party.
"The Good Republican" is there to say that his party has become a bunch of crazy unreasonable fanatics. And if he doesn't say it in so many words, he lets the Democrats say it for him.
McCain filled this role, until he actually became the nominee. Huntsman briefly filled it. Now it's on to Christie.
Are the Dems really worried about a challenge from Christie? I suspect not, otherwise they would have made more of an effort to take him down. He certainly wouldn't still be getting the kid glove treatment long after Obama got his Sandy photo ops. The reports that Romney's team found issues with his background are not implausible. They sync with a Democratic establishment that believes that Christie isn't a threat and that they can destroy him any time they want.
Even if there is no ticking time bomb, considering what the Democrats did to a liberal Republican who was experienced and a war hero or a young female governor who had been praised for her reforms, I doubt they're too worried about an overweight governor whose big selling point is yelling at people in viral videos.