How do you see Trump as a threat to Romney? I don’t see them as having the same voters at all.
On the other hand, if Trump turned out to be a Palin Democrat, that could be extraordinary.
Part of Romney's attraction is his business knowledge and experience. Trump undercuts that support base. His potty-mouth straight talk gives real legitimacy to the outsider claim, rather than Romney's halfhearted one as a former governor of Massachusetts. And Trump undercuts the non-Evangelical support that Romney's getting. Could Palin claim a lot of these voters? Sure. Can she get them in the first few primaries, especially the open primaries? Don't think so, unless someone splits off Romney's votes and evens out the field.
I think it's a three horse race with Trump in it, or a one horse with Romney.