Starting this with a regular caveat: I am a Cruz guy. I like Cruz, and I hope he wins. I think he would do the best job at dismantling our ruinous regulatory regime, I think he would be the most staunch supporter of socially conservative values, I think he would make the best judicial nominations, I believe he is the most likely candidate to govern the way he runs, and I believe he is likely to win the general. I like Rubio and would be happy to vote for him, I just like Cruz more.
That said – since Iowa Trump has lost 6.3 points in the RCP national average, and Rubio has picked up 7.8. Pretty clearly Trump’s loss (plus a little more) has gone to Rubio. However, keep in mind Trump has had a floor and a ceiling, the range between is “soft support.” I believe that Rubio’s pick has been the soft support, and it would not be until Trump drops that the final 27% or so of Trump’s “hard” support moves that Cruz makes his pickup. If you assume that Carson’s remaining support (strong evangelicals) and Trump’s “hard” support (strong anti-immigration and anti-politically correct) are most likely to move toward Cruz rather than the more electable Rubio then Cruz’ national support would move to a bit over 50%.
Granted, this is wishful thinking, but where do you think Carson’s support will go? Where do you think that Trump’s hard core anti-amnesty support will go? Cruz still has a path to the nomination, and it involves not losing to Rubio in South Carolina. If he does? Well then oh nightmare of nightmares, we nominate Rubio. Pretty good day when Rubio is your backup plan.