It will be a very long 90 minutes for Republican nominee Donald Trump at his second US presidential debate with Democrat Hillary Clinton at 9am today (Singapore time).

The pressure on the 70-year-old will be intense as the backlash from his lewd remarks in a 2005 video saw many in his party withdrawing their support of his candidacy. Not only must he parry attacks from Mrs Clinton, 69, but he must also show contrition to stop more Republican desertions.

"I've never seen a candidate walk into a debate with this much at stake," said Clinton ally James Carville. "He's overweight, he's old, he's tired and he's crabby. And he's going to have a very long hour and a half."