Just a quick comment on a recent Vox article:
But in a head-to-head matchup among Republican voters, Trump beats Rubio 57-43. That suggests that Trump’s ceiling, at least among Republicans, is far above his current 25 to 30 percent, and he may well benefit as weaker candidates drop out.
This actually makes sense given the current polling that we have seen in the Republican race. For most of the year, if you add up the combined polled support of Trump, Carson and Fiorina plus attribute some proportion of Ted Cruz’s vote as extraordinarily anti-Republican establishment vote, this faction is the dominant faction within the Republican primary electorate. The sum of Trump, Carson, Fiorina often is over 50% and usually closer to 60%. The favored Establishment candidates (Bush, Walker, Kasich, Christie, Rubio) have either dropped out, or combined can poll less then a quarter of the Republican primary electorate.
Primaries often see large swings as the differences between candidates are not stark. However it is easier for a person to switch support to a candidate who is running in the same cluster as the candidate that the person is switching support from, than for support to go to a candidate in a distant cluster. The anti-establishment lane in the GOP primary is bigger than the establishment, and one of the candidates can’t be pushed aside due to a lack of funds.
So when the question and answer space collapses to only a single establishment and a single non-establishment choice, people gravitate towards their closest option from their current preferred option. That means Trump is picking up all of his current support and the vast majority of the supporters of the other anti-establishment candidates while Rubio consolidates everyone else. And one faction right now is much bigger than the other.