If vanity is the soft underbelly of maleness, a vulnerable area between the armor of masculinity and the kill zone, it appears it was Donald J. Trump’s famous self-regard that was the target when Nikki Haley ramped up her attacks on her opponent in New Hampshire this week.

Soon after Mr. Trump appeared to confuse Ms. Haley in a speech with the former House Speaker Nancy Pelosi during a Friday night rally in New Hampshire, Ms. Haley pounced. Using a strategy out of the “Mean Girls” playbook (chapter heading: “The Underminer”), Ms. Haley, the former South Carolina governor and former ambassador to the United Nations in the Trump administration, wondered aloud at a news conference whether the former president would be “on it’’ enough to lead the nation.

“My parents are up in age, and I love them dearly,’’ said Ms. Haley, conjuring an image of doddering geezers rocking on the porch. “But when you see them hit a certain age, there is a decline. That’s a fact — ask any doctor, there is a decline.’’

It’s well established that many factors, both social and genetic, affect how and when we decline and even at what rate. Yet few barbs have more consistent power to sting the vanity of men at any age than the ones that cast doubt on their potency — real or imagined, mental or otherwise.

“There are so many legitimate faults, so much valid ammunition Haley could use in her fight against Trump,’’ said Sari Botton, editor of Oldster Magazine, a digital publication about aging. “Why stoop to bullying with ageist tropes about aging?’’

The answer is simple. The body has been centered in American political life ever since Mr. Trump, now 77, first tossed his hat in the ring back in 2015. Most often the body was that of a woman: dissected, derided, skewered by Mr. Trump with unvarnished glee.

What is new here is not merely the inversion of hoary sexist tropes but the zeal with which Haley, 52, now among the last remaining contenders for the Republican nomination, has seized on any opportunity to showcase her own vigor and contrast it with that of an aging male body — which, in the event of a Trump face-off with President Biden, would be the only kind. (It is worth noting here that all the Republican candidates have taken potshots at President Biden’s age and fitness while sparing Mr. Trump from similar attacks.)

Consider the choreography of Ms. Haley’s hyperkinetic hand gestures, the manner in which she jabs at the air, motioning in broad sweeps, swinging her arms in wide arcs, frequently using thumb and forefinger as if to snatch thoughts from the air. She keeps viewers’ eyes riveted on her at all times, a strategy seldom seen among high-profile female politicians, who are coached to keep their movements calm and tight. Not for Haley, the so-called Merkel rhombus, that serene hand gesture forever associated with the former German chancellor. Joining thumbs and forefingers at her body’s midpoint, Angela Merkel routinely created what resembled an energy-centering diamond: a power mudra. Ms. Haley, by contrast, is a human fury and never more than when she is on the attack.

Underlining an aging opponent’s cognitive shortcomings is surely a “cheap shot,’’ as Ms. Botton of the Oldster said. And it is one that may do little to help her recover from a resounding loss in New Hampshire. Still, Ms. Haley’s age-baiting strategy has at least one instructive effect, which can be clocked every time Mr. Trump reverts to mispronouncing her name. Does anyone believe Mr. Trump thinks Nikki Haley is called Nimbra, his garbled version of Nimrata, her birth name? Or is it just as likely that his supposed error is a tell for wounded vanity?