Donald Trump’s successful primary campaign was predicated on the idea that the GOP needs a new messenger and a new message.

Mr. Trump proved that he was right—as I well know, since I was often in his target sights! He successfully tapped into the anger and deep distrust that voters feel toward Washington. These voters believe the American dream is increasingly out of reach. They believe our system is skewed in favor of the powerful and the connected. They believe that the politicians elected to right these wrongs have only...