Regardless of what the Rubio campaign might say, they desperately need to clear 15% on Monday. The very least they must accomplish in Iowa is to create clear separation between themselves and the Christie/Jeb/Kasich pack that is all trying to finish second in New Hampshire. If he finishes at 10 or 11%, in a virtual tie with Christie or someone else for third, he will enter New Hampshire from a position of weakness, not a position of strength, and his chances at the nomination will plummet.
If, on the other hand, he finishes in the 15-20% range, he heads to New Hampshire with a convincing case that he is the sole feasible alternative to Trump and Cruz. And if he uses that boost to finish ahead of Christie/Jeb/Kasich, his campaign remains viable going forward.
The Rubio camp feels, based on recent polling and anecdotal evidence, that they may actually be in position to pull off a stunning upset on Monday by either beating Cruz or by cracking the 20% mark and finished in a virtual tie with Cruz.
The polling, which should always be taken with a grain of salt, provides reason to hope for Rubio's backers. The most recent poll taken in Iowa shows Rubio getting a surprising 18% of likely Iowa caucus goers. A Monmouth poll earlier this week likewise has him at 16%. PPP is a garbage troll organization but even they have him at 14%. Overall, in the last three weeks, Rubio has halved Cruz's lead over him in the RCP poll average. There's also a feeling (shared among several of the campaigns) that Cruz was materially damaged by Thursday's Fox News debate, to an extent that we don't really know yet.
Some portion of Cruz's support has always been about loyalty to Cruz. Some of it, though - especially in Iowa - has been about the belief that Donald Trump must be stopped in Iowa, and Cruz was believed to present the best shot of that. As Cruz' standing in the polls has slipped, this might well be generating an avalanche effect in which people leave Cruz not because of anything he's done, but because he's perceived as slipping in his chances to beat Trump.
The polling still isn't to be treated as gospel, even after all we've learned, and Iowans can definitely break late. But today's Des Moines Register poll (expected to be released around 5) may provide the Rubio campaign with a PR boost as they claim that they are now in the best position to stop the Trump train.
And if Rubio is able to sneak up and surprise Cruz in Iowa, that may change the dynamic of the whole race in ways that no one can yet predict.