1. South Carolina really demonstrated the narrowness of Cruz’s path to the nomination. For Cruz to win, he is going to have to shake loose the (self-identified) Evangelical and ideologically conservative voters who are sticking by Trump in order to send a message. The only way Cruz even has a chance is to show these voters that their champ is a chump.
That means that Cruz needs to lay a relentless beat down on Trump for the rest of the campaign. The model is what Romney did to Gingrich between the South Carolina and Florida primaries. There are several reasons why this will prove harder for Cruz than it proved for Romney.
The dynamics of the 2012 cycle were that the majority anti-establishment candidates always colluded with Romney to target the top Romney challenger. That meant that Romney had a clean shot at Gingrich and sometimes even had support. The dynamics of the 2016 cycle are for most of the candidates to leave Trump alone or else target the top anti-Trump. Rubio could potentially benefit from a Cruz–Trump spat, but it is also in Rubio’s interest to heckle Cruz while leaving Trump alone. It would be an act of simple public spiritedness for Rubio to primarily target Trump in the upcoming debate. There is no reason to think that Rubio will prioritize his internationalist (in the good sense of the term), interventionist (for better and worse), humanitarian, compassionate principles over short-term politics. I’m not complaining. Cruz wouldn’t have put aside short-term tactical advantage either. Don’t hate the player. Hate than none of these people are big enough to rise above the game.*
And, from Cruz’s perspective, it doesn’t matter. The interests line up in such a way that he has to fight a two-front war against Rubio and Trump. He can either fight it out and probably lose or not fight it out and definitely lose. The only way Cruz wins now is if he leaves a smoking crater where Trump used to be, while holding off Rubio. It is as simple as that.
2. The best argument against Rubio is that a Rubio nomination would inflame the arrogance and sense of entitlement of the Republican office holder/lobbyist classes. See? It all worked out in the end. Well, no. You are part of a coalition in relative demographic decline, and that coalition looks like it is about to fly apart. Too many elite Republicans will look at a Rubio nomination as proof that Republicans just need to follow the sage advice of the same wise men that led us to a place where Trump won the first two Republican primaries.
* Don’t bring up Bush. Bush went at Trump in debate, but Bush’s super PAC primarily targeted Rubio.