What if Clinton has to step down?
WHAT happens if a candidate for the White House is forced to quit the race?
Who steps up to fill his or her shoes?
It is a question suddenly under the spotlight 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 yesterday 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 United States 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.
The Democratic Party bylaws state "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 process.
At the Democratic Party gathering, the decision would be based on a majority vote by those present.
But there is little precedence to guide their choices.
Although Mrs 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 her running mate Tim Kaine, grassroots darling and her primary challenger Bernie Sanders and current Vice-President Joe Biden.
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.
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 Mrs Clinton would exit the race over health concerns.
"I don't expect Clinton to step down.
"She has an illness that is treatable," Mr Lublin said.
Added Ms Zaino: "This has been such a crazy election season... but that is something I am not expecting at all unless her health is far worse than we have been led to believe."
Her departure from the 9/11 ceremony, 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 fuelled speculation and conspiracy theories on the Internet, already awash with unsubstantiated rumours she may have a brain tumour, Parkinson's or dementia.
The root of the persistent claims about her 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. She later received the all-clear.
In recent US presidential history, only one candidate has dropped out.
Senator Thomas Eagleton, running mate of Democratic nominee George McGovern in the 1972 race, was forced to quit after it was revealed that he suffered from depression.