“Don’t blow this” is what Clinton hears most often these days, she told me, or variations thereof. As it has turned out, Clinton, who began her campaign intent on breaking the last barrier — the glass ceiling — has found her most compelling rationale in her own role as a barrier, a bulwark against the impossible alternative. As I was leaving our interview, she smiled, looked me in the eyes and left me with a casual reminder. “As I’ve told people,” she said, “I’m the last thing standing between you and the apocalypse.”