When will Trump lose the lead permanently

[Mr. Morden]

He's hardly invincible and yet nothing seems to have done him in yet so I'm curious as to when he's going to go down and what is going to cause it.

He doesn't actually have to go down in order to lose the nomination.  He just has to not gain much more when other candidates drop out.  30% isn't enough to win the nomination.

[Mr. Morden]

Dean still held onto his national polling lead even as his Iowa lead collapsed, didn't he?  (Not losing it until after he lost Iowa.)

Yeah, my hunch is that January will see some crazily large shifts in Iowa and New Hampshire polling as caucus/primary day approaches (as voters in those states near judgment day, and as they start to get bombarded by non-stop ads), but that Trump will hang onto his national polling lead, not losing it until some time after the primaries start.  (Whether that happens after Iowa, after New Hampshire, or later, I don't know.)

I guess a key moment will be on Iowa caucus night, when we see how many of Trump's "unlikely voters" show up.