Right now the buzz on the ground is that Rubio is drastically over performing expectations. No one really knows whether it will be enough to push him over either Cruz or Trump for one of the top two spots, but it is starting to look like a replay of 2012 where we could essentially be a three-way tie for first, which would be a major win for Rubio (and might also be a major win for Cruz, if he ends up ahead of Trump).
Where are these votes coming from? One of the most bizarre phenomena I observed yesterday at the Rubio rally I attended in Cedar Rapids was that there were people there who said that they had narrowed down their final choice to being between Rubio and Trump. I previously did not believe that such a person existed in America, as all the Trump supporters I have ever known have basically treated Rubio like he was infected with a particularly virulent strain of leprosy.
But I think that broad brush analysis ignores the fact that not all of Trump's support is hardcore Trump support. Some people have glommed on to his campaign because they believed he represented the best chance to beat Hillary, and they also saw him as somewhat inevitable. Rubio has been working the state tirelessly over the last two weeks attacking that message specifically and positioning himself as the candidate that Hillary is (justifiably) afraid of.
Rubio is doing better than expected, and it does not look like he is taking votes away from Cruz. Cruz looks to actually be also performing better than expected, and Carson is about where he expected to be as well. It's starting to look like Rubio might well have taken some support from Trump.