What happens if a US presidential candidate drops out?

It's a question suddenly under 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 re