If Donald J. Trump decides as president to throw a whistle-blower in jail for trying to talk to a reporter, or gets the F.B.I. to spy on a journalist, he will have one man to thank for bequeathing him such expansive power: Barack Obama. Mr. Trump made his animus toward the news media clear during the presidential campaign, often expressing his disgust with coverage through Twitter or in diatribes at rallies. So if his campaign is any guide, Mr. Trump seems likely to enthusiastically embrace the aggressive crackdown on journalists and whistle-blowers that is an important yet little understood component...