Donald Trump got sound advice the other day. At a rally at Davenport, Iowa, he told the crowd that a prominent supporter had called and urged him not to sweat all the attacks at the Democratic National Convention.

“Don’t hit down,” the supporter urged, according to Trump. “You have one person to beat. It’s Hillary Rodham Clinton.” By Trump’s account, he conceded the good sense of this, although he noted how he always prefers hitting back — “it makes me feel good.”

If so, he must have enjoyed his weekend. He spent it attacking not just Khizr Khan, the Muslim father of a soldier killed in Iraq who spoke at the DNC, but his wife. In other words, roughly 48 hours after publicly sharing the advice he had gotten not to punch down, Trump delivered a flurry of downward blows the likes of which we haven’t seen from a presidential candidate in memory.

The old political and media rule is unassailable. When you are the bigger, more famous figure, you only draw more attention to a less prominent critic by engaging. If people hadn’t heard, or heard about, Khan’s short speech against Trump at the DNC before, they probably have now.

In its unadorned righteous indignation, the Khan DNC speech was a stinging rebuke of Trump — Khan suggested the Republican candidate hasn’t read the Constitution, nor ever sacrificed anything for the country — and the mogul duly acted stung.

His first swipe was at Khan’s wife, Ghazala, for standing silently at her husband’s side during the speech (perhaps, Trump implied, she was forbidden from speaking as a woman?). In subsequently trying to tamp down the controversy, Trump stoked it further by saying Khizr Khan had “no right” to criticize him as he had and complaining about his viciousness.

The Trump response predictably fueled an all-out media blitz by the Khans. It validated one of the main lines of criticism of Trump at the DNC — that he is so thin-skinned, he can’t be entrusted with the awesome powers of the presidency. And his religiously fraught slap at Khan’s wife and his rhetorical manhandling of a family who had sacrificed so much for the country reinforced the sense that he refuses to honor basic political norms.

It’s not that grief validates a particular point of view, or someone who has suffered a terrible loss should be above criticism. But the grieving mother or father deserves an extra measure of respect. This isn’t just Politics 101, but Decency 101.

President George W. Bush was gentle with Cindy Sheehan, the gold-star mother who became a fierce critic of the Iraq War. Asked on “Fox News Sunday” about two parents of State Department employees killed in the Benghazi attack who have criticized her — including Patricia Smith at the Republican National Convention — Hillary Clinton said first, “My heart goes out to both of them,” and then countered their criticisms without making it personal.

This isn’t hard. Trump may figure he needn’t bother because he has weathered so many other controversies that appalled critics on the left and the right. But the playing field is different when he is potentially three months away from being elected president of the United States, as opposed to a Republican primary contender among many others. It is one thing to beat Ted Cruz and his family about the head and shoulders — he’s just another pol — but something else entirely to do it to the parents of an exemplary young man who sacrificed his life protecting others in Iraq.

Trump believes, from his decades in the public eye in the media capital of the world, that it always pays to be on the attack. This isn’t true anymore. The question no longer is whether he can garner headlines, but whether he can demonstrate his suitability to becoming commander in chief. The only one he’s hurt by his assault on the Khans is himself.

Rich Lowry can be reached via e-mail: comments.lowry@nationalreview.com.