NEW YORK (AFP) - On the skids in the polls and under fire for not releasing his tax returns, Donald Trump's campaign is sharpening a new attack on Hillary Clinton: Claiming she is not healthy enough to serve as president.

The Internet is awash with conspiracy theories claiming that she may have a brain tumour, Parkinson's or dementia, or complaining that she has "seizure-like facial expressions" or allegedly twitches.

Trump, who at 70 is 16 months older than his Democratic adversary, his spokespeople and surrogates have used innuendo and fabrication to peddle their theories that she is not physically or mentally fit.

"#WheresHillary? Sleeping!!!!!" he tweeted over the weekend.

Last week, he told voters in Iowa that Clinton was "not strong enough to be president." On another occasion, he maintained that she "importantly also lacks the mental and physical stamina to take on ISIS, and all of the many adversaries we face."

Former New York mayor Rudy Giuliani, a Trump supporter, stepped up the attack on Monday, telling Fox News that Clinton was "tired" and "looked sick." Over the weekend, he came under fire for telling viewers to google Clinton's health.

"Go online and put down 'Hillary Clinton illness,' take a look at the videos for yourself," he said on Fox News.

Trump spokeswoman Katrina Pierson, who is not a doctor, last week diagnosed Clinton with dysphasia, a disorder that impairs speech and comprehension, and pushed conspiracy theories about her health promoted by some conservatives.

Team Clinton vehemently denies any health concerns, and released a letter from her doctor in July 2015 giving her an "excellent" bill of health.

But the root of the conspiracy theories lies in 2012, when Clinton was nearing the end of her stint as secretary of state and a stomach virus and dehydration prompted her to faint, causing what her doctor said was a concussion.

They said they found a blood clot on the brain and Clinton complained of double vision. She appeared in spectacles featuring a prism when testifying before Congress on Benghazi in January 2013.

But she was later given the all-clear. Her test results were normal, and her cancer screening evaluations all negative, said a letter released by her physician Lisa Bardack last year.

Health is a legitimate subject of debate in presidential elections - Americans want to know that the person they are going to elect to one of the toughest jobs in the planet is up to the task both mentally and physically.

But history is also peppered with coverups. The public had no idea John F. Kennedy had Addison's disease. The severity of Woodrow Wilson's stroke was concealed in 1919, as were concerns about Franklin D. Roosevelt's health before his fourth re-election - just months before he died in 1945.

Ironically enough, Trump would be the oldest person ever elected US president if he wins. Clinton would be the second oldest after Ronald Reagan.

In 2008, then aged 71, Republican nominee and cancer survivor John McCain released more than 1,500 pages of medical history in a bid to lay to rest suggestions that he was either too old or too unhealthy to serve as president.

Jeanne Zaino, professor of political science at Iona College in New York, says Clinton may follow suit in releasing more medical records or have her doctor continue to reiterate her good health.

"I'm not sure it's going to hurt her but it does allow them (the Trump campaign) to come up with a counter to his tax issue," she told AFP.

What is clear is that running for president is grueling. Journalists trailing in Clinton's wake endure punishing schedules and little sleep.

The 68-year-old grandmother of two looks remarkably fresh faced and distinctly more robust than her husband, former president Bill Clinton, who underwent heart bypass surgery in 2004.

If Trump overplays the health card, it could backfire among women voters, who already have a very low opinion of the Republican nominee.

"Stamina, the word he keeps using, has gender overtones," Zaino said.

"To act as if this is a weak, frail woman who doesn't have the ability, the evidence suggests otherwise, and the same thing with him," she said.

"If you look objectively, it's hard to make the case that either one of them are suffering from lack of stamina."