> A friend of mine likes to point out that while Trump has a lot of polls with him as their top pick (among the Republican field, I presume), there are far fewer with him as their second or even third pick.
Polls rarely (if ever -- I can't remember ever seeing one that did) actually ask for and track second and later picks, and even if they did, I don't think there is much basis to expect that they are stable and predictive.
> As other candidates start to drop out, those that remain will start to gain the "second" and "third" pick supporters while Trump will remain with relatively the same amount of supporters.
That's not an implausible guess, though the motion since there has been any attention on the race has mostly been from other candidates to Trump.
> I think in the end, it will be hard for Trump to keep up the support he has in crowded field against a more narrow field with saner candidates.
Lots of people -- many of them non-Republicans or Republican establishment figures -- have said things like that, and it may be true, or it may be people projecting their own preferences and perception of what constitutes political "sanity" onto the Republican primary electorate.