Donald Trump tweeted on Tuesday that “the shackles have been taken off” his campaign — a thinly veiled threat suggesting that the final 28 days of the presidential race will be filled with personal attacks. That scorched-earth strategy is on full display in the Trump campaign’s latest ad, which targets Hillary Clinton’s health.

“Our next president faces daunting challenges in a dangerous world,” a voice-over says at the beginning of the ad. “Iran promoting terrorism; North Korea threatening; ISIS on the rise; Libya and North Africa in chaos. Hillary Clinton failed every single time as secretary of state. Now she wants to be president.”

The 30-second spot, titled “Dangerous,” then cuts to clips of Clinton coughing on the stump and being helped up the stairs and into her van.

“Hillary Clinton doesn’t have the strength, stamina or fortitude to lead in our world,” the narrator continues over an image of her checking email on her BlackBerry. “She failed us as secretary of state. Don’t let her fail us again.”

Clinton’s health — which has long been the subject of right-wing conspiracy theories — was put into stark relief on Sept. 11 when she abruptly departed a 9/11 memorial service at Ground Zero, and video emerged of her stumbling and needing to be helped by aides into her van.

Hours later, the campaign said that she had been diagnosed with pneumonia two days before and that she had become overheated and dehydrated at the event.

The episode forced Clinton off the campaign trail for three days to rest.

Trump did not initially attack Clinton’s dizzy spell, but the former “Celebrity Apprentice” star disclosed the results of his own recent physical on “The Dr. Oz Show” — prompting his Democratic rival to release an updated letter from her doctor.

“My overall impression is that Mrs. Clinton has remained healthy and has not developed new medical conditions this year other than a sinus and ear infection and her recently diagnosed pneumonia,” Clinton’s longtime physician, Lisa Bardack, wrote in the letter. “She continues to remain healthy and fit to serve as president of the United States.”

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In recent weeks, however, the Republican nominee has attacked Clinton’s stamina head-on, mocking her for taking time off the trail and mimicking her Sept. 11 stumble.

“You see all the days off that Hillary takes? Day off, day off, day off,” Trump said at a rally late last month. “All those day offs, and she can’t even make it to her car. Isn’t it tough? All those day offs. … Boom.”

But during Sunday’s presidential debate, Trump was asked if there was anything he admired about his opponent.

“She’s a fighter. I disagree with much of what she’s fighting for,” he said. “I do disagree with her judgment in many cases. But she does fight hard, and she doesn’t quit, and she doesn’t give up.”