If “Trump the Candidate” is often an outsize caricature of Donald J. Trump, the New York businessman, then “Trump the Debater” was the opposite of outsize: a subdued, scowling titan, stuffed into a black suit, straining at the seams as the debate at Hofstra University wore on. The answer to the core questions — could Mr. Trump comport himself as disciplined “Teleprompter Trump” without the aid of actual teleprompters, and could he appear plausibly presidential opposite Hillary Clinton — came early in the debate. And the answer, at least for the first portion of the debate, was yes. By the...