I am not sure it has been discussed on ge yet, so I wanted to bring it up here. Trump is polling at ~35%. So, supposedly he is "way out in front" but if a lot of the extra candidates started falling out, their support would start to funnel elsewhere. It is my belief it would go nowhere near Trump or Carson. That in mind, by the time Iowa comes around let's say we are down to 6 candidates. Will Trump's lead be anywhere near as big? Isn't it plausible that his numbers will stay stagnant ( they haven't ever really grown beyond where they are now) and his competitors will catch up?
I don't see it happening too soon because everyone still thinks they will win for some reason. (who is still whispering in Graham's ear that he has a chance?) But if Bush drops out you have to believe that his supporters and money go to Rubio. If that is the case, I don't see how he wouldn't rocket up to a solid second in the mid twenties. Then, you'll start seeing other candidates falter and that could continue.
It's coming down to Trump versus Rubio/Cruz. Even if all the other candidates fold, and most will, and even if all of their supporters go to Cruz/Rubio, and most will, Trump can remain competitive as long as Rubio and Cruz are splitting the rest of the vote and delegates.
What should concern all conservatives is not Trump himself, but Trump's supporters. I don't think Trump will go third party, but I do think there are a lot of people supporting Trump because they are furious at the Republican party's general ineptitude and betrayal on key issues like immigration. Many of these people will stay home on election day, which could very well lead to another Romney-style defeat - even to a candidate as lousy as Hillary Clinton.
Cruz is in a far better position to bring those disaffected conservatives back into the fold. Rubio will have far more appeal to independents. Which is more important? After seeing what happened to Romney in 2012, I gotta say that rallying the base and bringing in Rust Belt blue collar traditional Dems may be more important than chasing "Independents." Cruz might actually be the stronger general election candidate.