Death, declination or "otherwise".
Those are the scenarios under which the Republican presidential candidate can be replaced.
Rule 9 of the Republican National Committee's provisions lays out the rules for replacing a candidate if, for example, the candidate quits, dies or becomes incapacitated.
But an associate professor of American Politics at the United States Studies Centre in Sydney, Brendan O'Connor, says there is no provision for removing candidates against their will.
"Trump would need to stand aside, and he won't do that. There's no indication that the man takes the opinions of others who disagree with him very seriously, within his own party. And it's very, very unlikely that he will step down."
Increasing pressure is being placed on Mr Trump to step down in the wake of the latest scandal.
Insiders say there is talk of persuading vice-presidential candidate Mike Pence to leave the ticket.
But Mr O'Connor says that appears unlikely to happen.
"There could be all sorts of pressure that could be placed on Trump. Financial support will probably wane in the coming week or so. But all of these things, really, are just asserting forms of moral pressure on Trump, and he seems pretty immune to people saying, 'Look, you've got to change the way you talk, the way you behave.' Contrition is, you know, something Trump struggles with."
Making a change to the top of the ticket this close to Election Day would be without precedent in the history of modern US elections.
Ballots, which list Donald Trump, have already been printed, and early voting has already begun.
Mr O'Connor says, if a candidate died, for example, at this stage of the campaign, that person would then be replaced, most likely, by his vice-presidential running mate.
"So, typically, I think the vice-presidential running mate would be expected to be top of the ticket, and they would choose another vice-presidential running mate. So that would be the case. It's never happened. In 1912, the running mate of William Taft, who was president at the time, died, and that person was replaced, by Taft, with another running mate."
Even if, hypothetically, Donald Trump were to commit a major crime, it would not stop him from running for the presidency.
There is nothing in the constitution banning alleged criminals from running for office, and Mr Trump could possibly pardon himself after winning.
And Mr O'Connor says the most loyal Trump supporters are unlikely to be deterred from voting for him.
"A large amount of Trump supporters are going to support him no matter what he says or has done. And he made the statement earlier in the year, he said he could shoot someone in Fifth Avenue in New York and most of his supporters would still want to see him elected president. So that was an outrageous thing to say at the time, but, in retrospect, it seems pretty spot on."
The controversy over Mr Trump's latest comments has spilled over to politics in Australia.
In Canberra, Minister for Women Michaelia Cash and Prime Minister Malcolm Turnbull are standing united in their views.
Cash: "The comments were demeaning. They were disappointing, and they were wrong, full stop."
Turnbull: "And I would add to that they are loathsome and they deserve the absolutely universal condemnation that they've received."
But speaking on Channel Seven, crossbench senators Pauline Hanson and Derryn Hinch were not in agreement on it.
Hanson: "What he said was vulgar. I don't support it whatsoever. It was said, not on camera, it was said behind the scenes, and it was a tape recording. Now let's be honest about it, there are a lot of men out there that say horrific things, probably up to the same standard as what Trump said."
Hinch: "No, Pauline, I can't let you get away with that. A normal man, a normal man in a private conversation, would not talk about this. A normal man, Pauline, would never consider invading a woman's space so much that, in his mind or to his mates, he would say, I mean, that is sexual assault."
