I know people keep saying that we've only had 3 states vote and so there is plenty of time for the race to turn around for Rubio, who has yet to win a single state, especially now that he is getting establishment support. I really don't think so, the math is just very daunting. You just have to realize that by the end of March 15th, 59.9% of the delegates will have been allocated. Is there any scenario here where Rubio does well and gets large amounts of delegates when there will be four major candidates competing, Trump, Cruz, Rubio and Kasich (who says he is in the race at least until after the OH primary on March 15)?
Let's assume that Rubio runs the table after March 15th, taking every single state, including Trump's home state of NY and other northeastern liberal states where Trump should do well like NJ and CT (note, that in the latest MA poll, Trump is winning by 34 points, with 50% of the vote). Well, that's still 40% of the delegates, he also needs to get 245 more delegates before the end of March 15th in order to get to the magic 1,237 delegate number. Even getting to 245 (about 16.5% of the total) will be extremely tough. Most states are deep red states where Trump and Cruz should finish 1 or 2. Other states, like Massachusetts are very liberal and would likely go heavily towards Trump. Heck at this point it's not even clear Rubio is going to carry Florida on the 15th. While 245 delegates out of 1,480 doesn't sound like a lot, you have to understand that so far he has 10 out of 103, so less than 10% of the total allocated.
Regardless on whether than 245 is doable, we have to understand that there is zero chance of him running the table at the end. If you just eliminate NY, NJ and CT, which I believe are core Trump states, that takes 174 delegates off the table and makes him now get 419 by the end of March 15th or 28% of the total up to that point. I really don't see how he could possibly get that much in a 4 way race with lots of Trump and Cruz friendly states in the mix. Plus, winning with 419 still requires him to win in places like PA and WV (105 delegates total), which are heavily blue-collar and very Trump oriented. It also requires him to win Cruz friendly states like SD, MT and NE (92 total).
Note that Nate Silver estimates that Rubio will need to have 632 delegates through March 15th with is 43% of the total while in a 4 way race where states are apportioned proportionally. Again, I just don't see any realistic way that this could happen.