When President Trump declared on Saturday that reporters are “among the most dishonest human beings on earth,” it was not the first time he had disparaged the press. Nor was it out of character when, later that same day, his press secretary threatened “to hold the press accountable” for reporting truthful information that was unflattering to Mr. Trump. Episodes like these have become all too common in recent weeks. So it’s comforting to know that the Constitution serves as a reliable stronghold against Mr. Trump’s assault on the press.

Except that it doesn’t. The truth is, legal protections for press freedom are far feebler than you may think. Even more worrisome, they have been weakening in recent years.

The First Amendment provides only limited protection for the press. Over the centuries, courts have affirmed that it prohibits government censorship and offers some protection against defamation lawsuits. But journalists themselves have few constitutional rights when it comes to matters such as access to government sources and documents, or protection from being hounded by those in power for their news gathering and reporting. In those respects, journalists are vulnerable to the whims of society and government officials.

America’s press freedom, in other words, is something of a mishmash. There are some legal protections, but the press also relies on nonlegal safeguards. In the past, these have included the institutional media’s relative financial strength; the good will of the public; a mutually dependent relationship with government officials; the support of sympathetic judges; and political norms and traditions.