There's a lot to this. I too look at 76-year-old Trump and think: "At least the clock is ticking..."

And I don't see Christie as some Trump-slaying messiah who will bring him down for good. But I think he's capable of doing some real damage, while (importantly) puncturing Trump's aura of invincibility. The one thing I'm pretty confident of is that he *won't* slip into the futile old strategy of "Maybe if we just ignore Trump and avoid poking the bear, he'll go away..."

And the battle isn't for Trump's hardcore supporters. They're not leaving Trump, and will in fact double down if they see him attacked. The battle is for the many Rs out there who don't particularly want Trump but maybe think he's inevitable—or just dislike the Libs enough to rally around Trump in the name of supporting the team and spiting the other guys. I think the key is wounding Trump enough that he no longer looks so strong (much less inevitable)—thus opening up some space for other candidates to at least compete.