One way to know Donald Trump’s onslaught against Hillary Clinton’s “stamina” is working to his benefit: Nearly every national newspaper columnist and cable talking head is either calling him an idiot or a monster for doing it.

Mainstream journalists and commentators were tickled when Trump dubbed his former Republican rival Jeb Bush “low energy,” but since he’s moved on to Clinton, the humor seems to have worn off.

In recent days, Trump has said Clinton “lacks the mental and physical stamina” to deal with ISIS; that she “gives a short speech, then she goes home, goes to sleep”; and that she “takes a lot of weekends off.”

That Trump is mocking Clinton for having bedhead is too much for the political-media class.

The Washington Post actually fact-checked his statement on her stamina, giving it the paper’s worst rating for what it deems to be a lie. “Trump has claimed twice, without proof, that Clinton lacks the physical and mental stamina to be president,” the ruling said. “In the absence of any evidence, he earns Four Pinocchios.”

Curious that the Post, in earnest, would fact-check Trump’s opinion on his opponent’s energy level. The paper didn’t bother to investigate the veracity of Clinton’s claim in late May that Trump “lacks the temperament to lead our nation and the free world.”

In the absence of any evidence that Trump “lacks the temperament” to lead the country, she earns four pillows.

Liberal columnist Ruth Marcus, emotionally affronted by Trump’s attacks, said they have “no basis in reality, and no place in a presidential campaign.”

Except discussions about the personal health and physical fitness of someone who is or wants to be president always do — in every campaign, ever. It’s why candidates are expected to release new medical evaluations at some point during their campaigns.

It’s why the public suffered through a seemingly endless stream of articles wondering if John McCain, 71 years old when he ran in 2008 against Barack Obama, was about to die any moment. (“Is John McCain too old to be president?” asked the Associated Press.)

It’s why there’s still a debate about whether Ronald Reagan was mentally all there during his second term. (Bill O’Reilly’s 2015 “Killing Reagan” book is the latest salvo.) Time magazine ran a cover photo of Bob Dole in 1996 overlaid with the text: “Is Dole Too Old For the Job?”

Hillary released some of her medical information last year and it turns out she takes medication for an underactive thyroid, one of the main symptoms of which is … fatigue.

But Trump isn’t allowed to call her sleepy.

“If you don’t have a medical degree, shut up,” Bloomberg Politics reporter John Heilemann said of Trump and his supporters who have echoed his hits on Clinton.

MSNBC’s Chris Matthews said the attacks were “based on no information at all, he’s just saying it.”

It’s never a problem, though, for the media to regularly evaluate Trump’s mental status.

Earlier this month, Kathleen Parker of the Washington Post wondered if Trump has brain damage or “atrophy” from old age.

“If so, then this would help explain his impulsiveness, his inappropriate language, his quick temper and a ‘mean’ streak,” she said.

It’s actually Clinton who in 2012 passed out from dehydration and suffered a concussion.

But shh! That has no place in this campaign!

MSNBC host Joe Scarborough said on his show one day earlier this month that he spent the previous day getting phone calls “and everybody was asking me about [Trump’s] mental health . . . Everybody was calling me saying, ‘What’s happening with him? What’s wrong with him?’ ”

In June, New York Times columnist David Brooks offered that Trump may have an “inability to identify and describe emotions in the self.” The next month, he questioned whether Trump is a narcissist with “unstable self-esteem.”

None of these people, as it happens, is a licensed psychiatrist.

In this election, the media have deemed it entirely fair for journalists to render a diagnosis on Trump’s mental health. But it’s out of bounds for him to fight back by conjuring up images of Clinton in a nightgown.

If Trump has ever landed a perfect punch, this is probably it.

Eddie Scarry is a media writer for the Washington Examiner.

