For Hillary Clinton, the most politically damaging aspect of her recent health scare is not any new revelation but the re-emergence of an old pattern.

The image of her buckling at the knees Sunday pushed doubts about her personal well-being from the crazy realm of conspiracy mongering squarely into the mainstream of serious discussion.

Making things much worse, though, was her campaign's handling of the episode — the delays, the half-explanations, the grudging trickle of information — which played to deep concerns going back to Clinton's White House days and controversies over openness and candor.

Far and away the biggest impediment standing between Clinton and the White House is that a great many voters, including some with every intention of voting for the former first lady and secretary of state, simply do not trust her.

The events Sunday did nothing to reassure them.

Rather, they brought a rush of memories from the last two decades — of parsed words, of legal and ethical controversies, of tiptoeing to the edge of accepted rules — that are symptomatic of the political ailment known as Clinton fatigue.

Sunday's episode suggested that internet-fueled rumors about a physical and mental breakdown, though fanned by rival Donald Trump and his backers and seemingly far-fetched, need to at least be addressed.

For Democrats, it was maddeningly frustrating.

David Axelrod, the impresario behind President Barack Obama's election and hardly a fan of Trump, offered his astringent commentary on Twitter.

“Antibiotics can take care of pneumonia,” he said, referring to Clinton's belatedly announced diagnosis. “What's the cure for an unhealthy penchant for privacy that repeatedly creates unnecessary problems?”

Her campaign announced it would soon release additional information about the candidate's medical condition, going beyond a two-page letter attesting to her good health that her personal physician issued in July 2015. Aides insisted pneumonia was her only health issue.

For his part, Trump has issued a vague and hyperbolic four paragraphs from his doctor declaring the Manhattan businessman, if elected, would “unequivocally … be the healthiest individual ever elected to the presidency.” It turned out the doctor dashed off the letter in five minutes.

On Monday, the Republican nominee was uncharacteristically restrained about Clinton's malady, saying he had recently undergone a physical and would announce the results on Thursday's episode of the syndicated show hosted by cardiologist Mehmet Oz.

Clinton took ill suddenly while attending a New York ceremony marking the 15th anniversary of the Sept. 11 terrorist attacks. Reporters allowed to follow the candidate noticed her abrupt departure, but the campaign refused to answer questions about where she was going or why; in a breach of protocol, her motorcade left without them.

She emerged about 90 minutes later from her daughter Chelsea's Manhattan apartment, looking chipper and saying she felt fine but refusing to offer any details about her affliction.

Clinton said Monday that she was feeling better. She said she never lost consciousness and didn't think her pneumonia diagnosis was significant enough to disclose beforehand.

“I just didn't think it was going to be that big a deal,” she said. She told CNN's “Anderson Cooper 360” that despite a doctor's orders to rest for five days she thought she could “just keep going forward and power through it, and that didn't work out so well.”

The campaign initially told reporters Clinton had suffered from heat exhaustion. It was only after video surfaced of Clinton staggering into her van, with Secret Service agents propping her up, that the campaign released a doctor's note, explaining she had been diagnosed last week with pneumonia. (By Monday afternoon, the scene had been viewed more than 2.5 million times on YouTube.)

“I think that in retrospect we could have handled it better, in terms of providing more information more quickly,” Brian Fallon, a Clinton spokesman, said on MSNBC.

Opinions of Trump and Clinton, two of the least-popular candidates in presidential campaign history, are firmly held by most of the electorate.

So it is doubtful what happened Sunday will greatly shake up the race. But it does raise another hurdle for the Democrat among undecided voters and elevate the stakes for the first presidential debate in less than two weeks.

Associated Press contributed.

mark.barabak@latimes.com