Trump has an air of invincibility currently. However if he were to lose Iowa that meme and his bombast look hollow.
Trump knows this and is why he dropped hid Muslim bombshell the afternoon a single poll showed him behind Cruz, he is a great tactician IMO. Note how this am he walked back his attavk on Cruz. He saw and understood the backlash from MLevine and others to his attack.
Attacking Cruz’s faith will hurt him in Iowa major tactical error. Cruz has been endorsed by the major evangelical players in Iowa. Many of them know Raefael Cruz. Under the radar the attack on Cruz’s faith will be mentioned from the pulpit to churchgoers in Iowa.
This mistake may cost Trump big time in Iowa.
Moreover, I don't buy the notion of this "holy" voting base in IA. Iowa has routinely turned out jerk pols, like Grassley and Harkin. But even if Trump were to "lose" IA (impossible, given that it's a proportional caucus state) he'd still win delegates that Cruz can't get elsewhere.
Cruz isn't even on the radar in NC or GA or FL or NH. He's second or third, but distantly so I think, in SC and NV. In short, no. The "invincibility" is there because it's there, not because it's perceived to be there.