Donald Trump’s attacks on the news media since taking office have been so persistent, so over-the-top, so deranged—in a word, so Trumpian—that it’s not surprising to see a backlash. Journalists are denouncing Trump for his continuous stream of vitriol toward them. Newspapers and magazines are enjoying spikes in subscriptions and support. Around 200 activists even congregated in Times Square one Sunday in February to show solidarity with the press corps against Trump’s onslaught.

The press’s job of keeping the public well informed about national and international affairs is no doubt harder than ever today. Trump’s eagerness to attack the media relentlessly and without restraint has made it harder still. But the ultimate reason that a demagogue like Trump mounts such crude, broad-brush attacks is that the institutional power of the press has diminished, and Trump descries political advantage in putting more pressure on them.

In 2017, the news media command considerably less authority than they once did to set the news agenda, arbitrate disputes over public issues, or even establish a common standard of what we ought to be discussing. Seeing that weakness, Trump appears to believe that he doesn’t need the mainstream media the way his predecessors did—and that when it comes to raw, open warfare with the journalists who cover him, he has nothing to lose. Whether he is right or wrong will depend on the performance of the press in the next four years.

When we hear the young Trump administration described as “unprecedented” in its treatment of the media, we often forget that all presidents spar with the press, often viciously. Almost every administration in modern times—since the rise of an influential press corps in the early 20th century—has at some point been labeled the “worst ever” in its treatment of the fourth estate, only to have such florid overstatements debunked in the light of history.

But if there is one close parallel with Trump in recent presidential history, it is with Richard Nixon, generally considered the president most unfriendly to the news media until now. Like Trump, Nixon hated the press for two very different reasons, one political, the other personal. ...