Cruz's performance in South Carolina last night confirmed that for me. That he did not carry a single county including those heavily laden with evangelicals is not a good indicator that he can effectively compete in the general election.
Cruz got zero of the 50 delegates in South Carolina. There is no way you can spin that as a positive. It was a disastrous night for Cruz.
Trump is running strong in every corner of the nation. Even the one state he lost (Iowa), he was a strong second and only a few thousand votes (and one delegate) behind the leader. As nominee, he will likely win states that Republicans haven't carried in decades.