Ross Douthat predicts Rubio will be the nominee:

But I make that prediction gingerly, not boldly, because Rubio is a very strange sort of front-runner. He has never led a national poll. He is not cleaning up endorsements, nor raking in the cash: His recent fund-raising totals were weak given his seemingly-enviable position. Nobody seems impressed with his early state organization. He’s earned a round of favorable coverage after each debate without making much progress overall.

Douthat probably makes the best case for Rubio that anyone has made, but all the things he acknowledges in this quote point to why it won’t happen. Rubio is a “very strange sort of front-runner” because he is not even close to being the front-runner by any objective standard. Provided that one finds an excuse to eliminate every other candidate, Rubio becomes the de facto front-runner by virtue of being the only one left, but it doesn’t work that way. He isn’t impressing many party actors enough to win their backing, and he isn’t getting much grassroots support, either. Rubio’s campaign is consistent with what one might expect from a one-term senator’s long-shot bid, because that is what it is. Unlike his competitors, however, Rubio continues to be judged not by what his campaign is doing or failing to do, but according to an entirely different standard that says he must be at or near the top of the field because that is where Rubio is “supposed” to be…because he’s Rubio. If front-runner status isn’t determined by polling, endorsements, fund-raising, or campaign organization, the label ceases to be descriptive and instead becomes a way of saying, “This is the candidate that I hope will win.” If that’s what you want say, that’s fine, but that is very different from describing what’s happening in the race.

It is possible that Trump and Carson will fail to build the organizations in the early states that they’ll need to get all their supporters to show up for them, but then the same seems to be true of Rubio, and he has fewer supporters to start with. Rubio’s high favorability numbers among Republicans are often cited as proof that there is no strong resistance to his candidacy, but then Carson’s numbers are even higher. If Rubio’s personal story is compelling to many people, Carson’s personal story is even more so. As hard as it is to believe that the GOP will nominate either Trump or Carson, their supporters seem very loyal so far, and nothing that either of them says appears to alienate the voters that favor them. I doubt either of them could win the general election, but most Republicans in the relevant states don’t see things that way.

To believe that Rubio will be the nominee requires explaining why most of that support is going to evaporate in the meantime and how that will redound to Rubio’s benefit. No one has provided such an explanation, and I don’t think there is one. The conventional wisdom is that Rubio’s relative advantage over these candidates is that he is more fluent in talking about policy issues, but that isn’t really an advantage with the people that support the two leading candidates. Trump and Carson supporters are not at all concerned that their preferred candidates don’t know much about policy. For some of them, this may even be a plus. That doesn’t bode well for a candidate whose main claim to being a plausible nominee is his supposed policy expertise.

The case that Rubio will be the nominee depends heavily on the fairly rapid implosion of at least the top two leading candidates, and it further depends on believing that many of the supporters of the non-politician insurgents will rally behind someone who has spent his entire career in politics and now works in Washington. Beyond that, it also assumes that most of Bush’s donors will decide to write him off in the near future and throw their support behind Rubio, whose candidacy will have been partly responsible for undermining Bush’s chances. Supposing that Bush donors do decide to dump their candidate and go to someone else, why would they choose to reward Rubio for his role in Bush’s defeat?

Rubio isn’t the front-runner now, and it’s very unlikely that he will ever be in that position.