In interviews and at campaign events throughout December 2015, Republican presidential candidate Donald Trump attacked Democratic rival Hillary Clinton, saying she lacks the “strength” and “stamina” to lead the country. (Sarah Parnass/The Washington Post)

For as long as Hillary Clinton has been in the public eye, she has answered questions — and sometimes volunteered information — about how much and how hard she works to get it all done.

Few, even her political enemies, have questioned her work ethic or staying power — until Donald Trump.

“She’ll do a couple of minutes in Iowa, meaning a short period of time. And then she goes home,” the GOP presidential front-runner said in Davenport, Iowa, a few weeks ago, as his attention turned to those areas. “You don’t see her for five or six days. She goes home, goes to sleep. I’m telling you. She doesn’t have the strength. She doesn’t have the stamina.”

Ever since — and increasingly in recent days — the magnate has lobbed a barrage of insults at Clinton from onstage at his campaign rallies, on television and online. The former secretary of state is “low-energy,” Trump says. She lacks stamina. She’s physically weak.

The attacks — often coded, always personal — seem to be aimed at raising questions in voters’ minds about a factor that has long been whispered in some GOP circles: how Clinton’s age could affect her ability to serve.

The regularity of Trump’s attacks on Clinton’s alleged physical weakness suggest that the magnate thinks he has touched on a legitimate campaign failing.

Speaking last month about an apparent bathroom break that delayed Clinton’s return to the stage during the Democratic debate sponsored by ABC News and the New Hampshire Union Leader, Trump looked to draw a sharp contrast.

“I think that my words represent toughness and strength. Hillary’s not strong. Hillary’s weak, frankly. She’s got no stamina; she’s got nothing,” the billionaire said on NBC’s “Meet the Press.” “She couldn’t even get back on the stage last night.”

Her energy level, he has said, should disqualify her from the presidency. “Hillary is a person who doesn’t have the strength or the stamina, in my opinion, to be president,” Trump told ABC’s “This Week.” “She doesn’t have strength or stamina. She’s not a strong enough person to be president.”

Trump, who often takes credit for saddling former Florida governor Jeb Bush (R) with a “low-energy” label, has lately used the same line on Clinton.

“She’s even lower-energy than Jeb Bush,” he told a South Carolina crowd on Wednesday.

After launching her second presidential campaign, Clinton kept a lighter public schedule than many rivals for most of the year. But as the Iowa caucuses and New Hampshire primary approach, her trail schedule is kicking into high gear. In the first week of 2016, Clinton has three New Hampshire events, five Iowa events and two appearances in Nevada in the space of four days.

Trump’s attacks have baffled some Clinton backers and angered others, with many of her supporters hearing more than a whiff of sexism in his words. For his part, Trump has accused Clinton of playing the “women’s card.”

Clinton may have laughed off Trump’s frequent jibes about her energy level and stamina, but her aides haven’t ignored them. They have taken pains to point out that she sat through an 11-hour House committee hearing on the Benghazi attacks in October, with exasperated facial expressions and a more clipped tone of voice as the only clear signs of fatigue after several hours.

Most recently, a campaign aide noted that Clinton seems to out-hustle her younger aides on the trail. The longer the day, “the more likely she is to want to have a beer with staff at the end,” Clinton’s communications director, Jennifer Palmieri, told Politico.

Among Clinton’s supporters, Trump’s swipes only add to the aura of mental and physical toughness that has long been a Clinton campaign staple, along with an accounting of her long political résumé. Her allies say they think the attacks may serve to bolster her appeal to women — especially those who have dealt with questions about their ability to keep up with men in the workplace.

“If they are young, people wonder if they have enough experience or the toughness to deal with the big boys in Washington. If they are middle-aged and have children, people wonder how they will run and serve and take care of children. If they are older, people wonder if they will have enough stamina,” Democratic pollster Anna Greenberg said. “People rarely ask these questions of men, let alone Trump, who is the same age as Clinton.”

Despite the fact that, at 68, Clinton is younger than 69-year-old Trump and 74-year-old Democratic rival Bernie Sanders, Clinton’s more than 25-year tenure in the public eye works to her disadvantage on this issue, said Jennifer Lawless, a professor of government at American University who studies women in politics.

“She seems like she’s been around forever, so it cues people to think more about her age than other candidates,” Lawless said. “That might be part of the reason she’s getting more attention than Donald Trump and Bernie Sanders for her age.”

Trump isn’t the only presidential candidate to raise criticisms of Clinton that hint at her age. A Web ad for Sen. Marco Rubio (R-Fla.) — who is 24 years her junior — featured Clinton’s recent comments that she is “from the ’60s — a long time ago,” to tee off Rubio’s mantra that frames the 2016 election as a “generational choice.”

At the last Democratic debate, former Maryland governor Martin O’Malley, 52, broke into an exchange between Clinton and Sanders to ask: “May I offer a different generation’s perspective on this?”

The attention Clinton’s aides have paid to anything that might suggest weakness or fragility dates to well before this campaign. It was especially true when Clinton fell and suffered a head injury in 2012, in the waning weeks of her tenure as secretary of state. Details were initially scant and closely held, but as her condition kept her away from work for weeks, aides provided extensive medical information about what was described as a temporary and relatively minor illness.

Clinton later said she was surprised by the illness, because she had always been healthy. Her campaign issued a medical fitness report last year, pronouncing her fully recovered and healthy overall. (Trump, who refers to his health as “perfection,” released a letter from his doctor last month in which the physician said that Trump’s “physical strength and stamina are extraordinary.”)

Meanwhile, in conservative media, Clinton’s illness was discussed as a sign of frailty and a potential liability in her 2016 campaign.

“This will be an issue in the 2016 race, whether she likes it or not,” Karl Rove, former deputy chief of staff to President George W. Bush, said in 2014, after suggesting that she might have a brain injury.

Clinton is slimmer and fitter now than when she left the State Department, a disciplined dieter who practices yoga in her hotel room, according to campaign staff. Her campaign mantra that she is a fighter who will work every day to address a growing list of causes and perceived injustices is a message that frames Clinton as possessing as much energy, stamina and work ethic as any younger person, or a man.

Trump’s implication that Clinton has little fire in the belly and would not work hard as president seems to have annoyed the Clinton team more than alarmed it. Her campaign declined to comment.

Those who know her best are skeptical that the attack will resonate, saying it has little merit.

“In all honesty, I cannot remember a time when this came up as an issue,” said Ann Lewis, a longtime Clinton adviser. “Because her reputation even among people who have opposed her on issues is: This is someone who does the work, does the homework and is prepared to stay as long as it takes to get the job done.”

Lawless, the AU professor of government, notes that Trump’s “stamina” comments could give Clinton a boost — they could play into her ability to identify with women, especially of a certain age, who have a sense that they have always been discounted and have had to work harder and prove more.

“There are women who are her age and who are still working and still feeling quite spry,” Lawless said.

Republican presidential frontrunner Donald Trump has publicly insulted at least 68 people or groups in 2015, many of them multiple times. Here is a comprehensive list. (Gillian Brockell,Thomas LeGro,Julio Negron/The Washington Post)