I think the reason Trump is keeping the lead is because his whole image is based around a joke, and he plays along. The other candidates might jump closer to him, get a bit more spotlight, and can be seen as the jokes they are...while they're trying to play serious. Anytime Carson/Rubio/Cruz get close to the spotlight they get hammered by some of the crazy shit they say, but Trump owns his crazy shit, so it doesn't work against him as much as it does others.
Though I don't see Trump really going UP in polls, ever. So whether he wins or not is going to depend on if the field stays big long enough. If Carson/Cruz/Rubio/Paul/Bush/Kasich all stay in the whole primary season Trump could still hold on. And it would be a glorious thing, seeing someone who half the GOP likely considers a total joke and they hate...representing their party in the national spotlight. I was just browsing some Trump articles on facebook since he's trending with his $5 mil shenanigans and on Fox News the most-liked comment is fairly anti-Trump (and even somewhat thought out, not just a crude attack). He doesn't have broad support, and the democratic party will clearly get behind Hillary a lot more than the R's will get behind Trump.
Of course if Trump is the R candidate there's no reason Sanders couldn't destroy him in a national election. But two more months to see if there's much hope in that.
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