On Oct. 19, as the third and final presidential debate gets going in Las Vegas, Donald Trump’s Facebook and Twitter feeds are being manned by Brad Parscale, a San Antonio marketing entrepreneur, whose buzz cut and long narrow beard make him look like a mixed martial arts fighter. His Trump tie has been paired with a dark Zegna suit. A lapel pin issued by the Secret Service signals his status. He’s equipped with a dashboard of 400 prewritten Trump tweets. “Command center,” he says, nodding at his laptop.