Washington (AFP) - What happens if a candidate for the White House is forced to quit the race? Who steps up to fill their shoes?

It's a question suddenly in the spotlight Monday as Democratic nominee Hillary Clinton battles a case of pneumonia that left her feeling so poorly she abruptly left a high-profile 9/11 memorial ceremony a day earlier.

The former secretary of state, 68, was resting at home in Chappaqua, New York and cancelled appearances at events scheduled for Monday and Tuesday in California.

"(Clinton) continues to feel better, but intends to remain at home today, following her doctor's recommendation to rest," campaign spokesman Nick Merrill said.

The US Constitution gives no guidance on instances when a candidate is unable to finish a political race.

The procedures are found by delving into the internal regulations of the political parties.

Article two, section seven of the Democratic Party bylaws states that "a special meeting to fill a vacancy on the National ticket shall be held on the call of the Chairperson."

Republican Party regulations describe a similar procedure.

At the Democratic Party gathering, the decision would be based on a majority vote by those present.

But there's little precedent to guide their choices.

Although Clinton is expected to make a full recovery from what appears to be a routine illness, analysts have floated names of those who could take on the party mantle should she be forced to drop out.

They are: Clinton running mate Tim Kaine, grassroots darling and Clinton's primary challenger Bernie Sanders, and current Vice President Joe Biden.

- Uncharted territory -

David Lublin, a professor of government at American University, said Democratic party officials could choose anyone who meets the criteria to serve in the nation's highest office.

"It would certainly would be uncharted territory," he said, adding that in his opinion, the most logical choice would be Kaine, followed by Sanders and the popular Biden.

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Jeanne Zaino, a professor of political science at Iona College in New York, said the parties have kept the process "purposely vague because it gives them the opportunity to make the best decision, rather than tie their hands with some kind of process that would give them a nominee they will not be comfortable with."

Both experts were doubtful that Clinton would exit the race over health concerns.

"I don't expect Clinton to step down. She has an illness that is treatable," Lublin said.

Added Zaino: "This has been such a crazy election season, I wouldn't been surprised if anything happened, but that is something I am not expecting at all unless her health is far worse than we have been led to believe."

Clinton's departure from the 9/11 ceremony at New York's Ground Zero on Sunday, captured on amateur video, showed her wobbling and being held up by members of her entourage as she got into a vehicle.

The episode has fueled fresh speculation and conspiracy theories on the internet, already awash with unsubstantiated rumors Clinton may have a brain tumor, Parkinson's or dementia.

The root of the persistent claims about Clinton's health lies in 2012, towards the end of her tenure as secretary of state.

A stomach virus and dehydration prompted her to faint, causing what her doctor said was a concussion. Doctors said they found a blood clot on the brain.

Clinton later received the all-clear.

In recent US presidential history, there's been only one case of a candidate dropping out.

Senator Thomas Eagleton, who was the running mate of Democratic nominee George McGovern in the 1972 race, was forced to quit after it was revealed that he suffered from depression.