Donald Trump, in a sharp shift in strategy, is now refusing to appear on many television outlets, and top advisers who want to limit his exposure are no longer notifying him of every interview request. According to sources familiar with the campaign, one faction is worried that the constant rounds of interviews entail too much risk of the candidate making mistakes or fanning minor controversies, even though his mastery at driving the media agenda helped power his Republican primary victory. A series of clashes over these and other tactical questions has caused Trump himself to openly question who is running...