Hillary Clinton is seen aboard the campaign bus in Cleveland on the third day of a bus tour through Pennsylvania and Ohio.

July 31, 2016 Hillary Clinton is seen aboard the campaign bus in Cleveland on the third day of a bus tour through Pennsylvania and Ohio. Melina Mara/The Washington Post

The former secretary of state, senator and first lady is the Democratic nominee for president.

Former secretary of state Hillary Clinton visits key states in her quest to become the Democratic nominee for president.

Former secretary of state Hillary Clinton visits key states in her quest to become the Democratic nominee for president.

Hillary Clinton’s campaign and her allies are planning an aggressive, sober defense of their candidate in response to businessman Donald Trump’s trademark personal attacks, which he has already aimed her way.

As he did with other candidates, “lyin’ ” Ted Cruz, “low energy” Jeb Bush and “little” Marco Rubio, Trump has slapped labels on Clinton that he hopes will stick: weak and incompetent.

Clinton’s allies believe that Trump is intentionally zeroing in on a character trait that they see as her key strength — her work ethic. They think the Republican front-runner is trying to neutralize that strength and turn it into a weakness, with insinuations about Clinton’s health, fatigue level and even her appearance. To stop Trump, the campaign and Clinton’s network of supporters have begun planning a swift and aggressive response, they say.

“There’s no doubt that this is one of his tactics,” said Sen. Amy Klobuchar (D-Minn.), a Clinton supporter. “He tries to portray someone in a way that he thinks is going to hurt them, and he does it very pointedly.”

Klobuchar added: “He seizes on things that he’s hoping that people are predisposed to think. That’s what really bothers me about this, because it couldn’t be further from the truth.”

In interviews and at campaign events throughout December, 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)

Adding to the challenge is a long-standing conservative narrative about Clinton’s health and stamina that began with her high-profile illness and fall in 2012, which left her with a concussion, at the close of her tenure as secretary of state. Clinton has fueled that narrative, even among some of her own supporters, with a series of stumbles that aides have attributed to the normal fatigue of a vigorous campaigner.

But these aides vehemently deny that there is any underlying stamina problem for Clinton — and they say that nothing could be further from the truth. They also say the accusation is sexist. They hope that, given Trump’s mounting troubles in the GOP nominating contest, the time is right for an effective counterpunch.

“I don’t think anybody else would get that question,” said Anita Dunn, a chief strategist for Barack Obama in 2008. “I think a lot of people are going to push back on [Trump] and call foul.”

“The purely sexist attacks on Hillary, the slightly veiled ones, are going to backfire [in the general election] in a way that they don’t in a primary,” Dunn added.

Clinton’s aides, who are well into their preparations for a possible general-election matchup against Trump, have begun executing a strategy to counter Trump’s pattern of personal insults by “going big” on policy and substance — and also by not being shy about noting her penchant for marathon workdays.

Aides have described in detail, for instance, how Clinton responded to the Brussels terrorist attacks as Americans awakened to the devastating news. After arriving in Seattle at roughly 2 a.m. local time for campaign stops later in the day, Clinton was awake and preparing to place a series of phone calls to the major television networks by 5 a.m., when coverage of the attacks was well underway on the East Coast.

Hours later came another set of on-camera interviews with cable television hosts. In each sit-down, a serious Clinton demonstrated her world-affairs acumen while also condemning Trump and other Republicans for their responses.

Republican presidential candidate Donald Trump remarked on Democratic presidential contender Hillary Clinton's brief absence from the debate stage on Dec. 19, saying: "Where did she go? I thought she quit." (Reuters)

“I don’t think we want to be inciting more fears,” Clinton said that day in an interview with NBC. “I don’t think we want to be playing to people’s concerns so that we turn against one another.”

“I think we have to have a slow, steady, smart, strong response, and we don’t need to be panicking,” she added.

Clinton’s press secretary, Brian Fallon, said the campaign will continue to be aggressive in countering what it expects will be a barrage of attacks from Trump.

“We know Donald Trump’s response to Hillary Clinton challenging him on his positions or his offensive statements is to sling personal insults at her, but that is not going to silence her,” Fallon said.

That formula has worked well for Trump. He has used it to fell opponents in the Republican nominating contest, including Bush, a former Florida governor, and Rubio, a senator from Florida.

“I was very rough on Jeb,” Trump said in an expansive interview with The Washington Post last week. It was “Jeb: Low-energy. Little Marco. Names that were devastating.”

[Interview: Trump predicts ‘massive recession,’ vows to eliminate U.S. debt]

Trump came up with his latest moniker for Clinton — “incompetent Hillary” — just as she began ramping up her criticism of his foreign policy knowledge. Before that, he accused her of lacking “stamina,” prompting a round of scrutiny of Clinton’s appearance and performance on the trail. Although Clinton’s allies say the labels have no basis in fact, they are aware that his insults have a habit of sticking.

“He throws these epithets at people, and sometimes they stick, sometimes inappropriately,” said J.B. Pritzker, a longtime Clinton supporter and donor. “And it’s demonstrably hypocritical to call her low-energy when his very first public dispute that got a lot of press was his unwillingness to sit through a two-hour debate.”

“I’ve never seen Secretary Clinton complain about the length of debates or about the length of her day,” Pritzker added.

Aides are also aware, though, that Trump is seizing on real optics.

He began harping on Clinton’s stamina at a time when she had been campaigning at an intense clip in her unexpectedly competitive primary fight against Sen. Bernie Sanders of Vermont.

[A scrappy Sanders campaign narrows Nevada delegate count six weeks later]

Her aides say that, since the Iowa caucuses in February, she has focused heavily on medium-size rallies and time-intensive, hand-to-hand retail politics — areas where aides think she is more likely to excel compared with the massive rallies that have become the bread and butter of both Trump’s and Sanders’s campaigns.

[Sanders, Clinton attack Trump and each other in Wisconsin]

All of it has taken a toll: Her grinding days begin early and end late, often leaving her on the verge of losing her voice or, in the midst of important speeches, coughing — then whispering — her way through her remarks.

Those moments have helped fuel questions about her fitness, even among supporters. At a January event, a woman stood during a town hall in Iowa to ask about rumors that she heard on television about Clinton’s health.

“They talked about how your health was ill, that you were really not equipped to be in the White House because of that,” the woman said.

Clinton laughed off the suggestion: “I’ll match my endurance against anybody.”

She also has had to navigate some real stumbles. The most damaging came after a week of campaigning and fresh off a cross-country flight to California for former first lady Nancy Reagan’s funeral. Clinton ignited an uproar among LGBT activists when she praised Reagan for “starting a conversation” about HIV and AIDS. Aides chalked up the mistake to fatigue, and they spent the next 48 hours working to reassure disappointed supporters and allies.

Aides characterized the gaffe as normal fatigue, and they don’t plan to make changes to her schedule based on Trump’s attacks.

Other allies say that Trump’s obsession with Clinton’s stamina is more than a little hypocritical.

As a case in point, they describe how Trump was in the midst of his longest-ever, 10-day hiatus from the campaign trail when he made this observation about Clinton in a recent interview with CNN: “I think she doesn’t have the stamina. You watch her life. You watch how she’ll go away three or four days; she’ll come back.”

It irks Clinton supporters that Trump, who is older than Clinton and has more grandchildren, could take shots at Clinton that hint at her age as a liability.

“Oh, I just think that’s silly. He’s older than she is,” said Hilary Rosen, a longtime Democratic strategist. “She doesn’t have to prove this to anyone. She’s been doggedly on the campaign trail for months with multiple events a day. And look what she did as secretary of state — downright Energizer bunny.”