Here's why. As a Bloomberg story points out, Trump can be tripped up, but you have to do it right:
Watch this famous David Letterman clip, when the talk show host slyly lures his prey to take the bait and then pounces.Do you see what happens here? Letterman doesn't give the game away. He seems to be doing a calm, polite interview about various subjects, including Trump's line of neckties and Trump's discussions with then-GOP presidential candidate Mitt Romney. But after Trump starts railing at China, Letterman --yes -- quietly pounces on the fact that Trump's ties are made in China. And Trump just shrugs.
Busted on his own rhetoric, Trump doesn’t know how to respond: He’s, for once, speechless.
Why doesn't Trump have a nasty comeback? Because Letterman doesn't betray hostility until he nails Trump. He doesn't give Trump a chance to get his testosterone flowing.
I live in New York. I know precisely what skill Trump has honed: When you encounter a direct challenge, you respond with even more aggression than was directed at you, and you accompany that with trash talk. Chances are your antagonist is concerned about maintaining a certain level of decorum. If you don't give a crap about decorum, but you actually seem to have your wits about you as you fire back (that's the trash talk -- this wouldn't work if you threw a punch), then chances are you'll back your opponent down, having shocked him by upping the aggression ante.
That's what Trump does. It works. And he gets to keep doing it because people who attack him generally don't creep up on him and then pounce, the way Letterman does in that clip.
Tonight, the candidates aren't going to try to go after Trump in that Letterman way -- they're afraid of Trump and won't attack him at all. The Fox moderators might go after him, but if so, they're going to go after him head-on. They want Classic Trump. They want an angry, nasty reply. (By "they," of course, I mean "Roger Ailes.")
Maybe they'll try to trip him up by asking a question that should be answered with a presidential level of attention to detail. But that won't work -- Trump will just veer off into chest-thumping generalities and never answer the question. He'll be unfazed.
In any case, no one's likely to get to him. His opponents are afraid to try, and Fox wants to keep The Trump Show on the air. And Trump knows that anyone who might want to try to take him will try to take him down directly, not stealthily, like Letterman. Trump is always ready for a direct attack.