It is hard to imagine a worse time that the illness could have struck.

Hillary Clinton's attendance at the anniversary commemoration of the 11 September attacks was meant to show her as America's strong and capable next commander in chief.

Instead as the names of the victims were read out, the Democratic presidential nominee left her place in the line of New York officials who stood in reverential silence and escaped ground zero with what little strength she had before she collapsed.

The episode on Sunday, which was made all the worse by video footage showing the 68-year-old being hoisted into a black van by secret service men, her feet dragging along the ground, has sent her presidential campaign spinning into full blown crisis.

One police official described her being thrown into the car "like a side of beef," losing a shoe under the vehicle.