Allan J. Lichtman, distinguished professor of history at American University, said the Democrats would not be able to hold on to the White House.

In the intervening weeks, the campaign was rocked by a series of events. The release of the Access Hollywood tape was followed by accusations from a growing list of women of various improprieties on Trump's part, ranging from verbal abuse and harassment to outright sexual assault.

At the same time, WikiLeaks released internal Clinton campaign emails, and the US government flatly accused the Kremlin of being involved. And let's not forget those presidential debates.

So plenty has changed. But one thing hasn't: Lichtman, author of Predicting the Next President: The Keys to the White House 2016, is sticking with his prediction of a Trump victory.

If you aren't familiar with his somewhat unique prediction system, here are the basics: The keys to the White House, he says, are a set of 13 true/false statements. If six of them are false, the incumbent party loses the presidency. His system has correctly predicted the winner of the popular vote in every US presidential election since 1984.

In September, he said the keys were settled enough to make an official prediction of a Democratic loss and a Trump win.

Your system for predicting the outcome of the election stays away from polls, electoral college maps and candidates' histories in favour of a more broad historical evaluation.

Lichtman: The 13 keys are a historically based prediction system that were founded on the study of every presidential election from 1860 to 1980, and I've since used them prospectively to predict, often well ahead of time, the results of all eight elections from 1984 to 2012. The keys basically assess the strength and performance of the party holding the White House. There are 13 keys. An answer of true on these true/false questions always favours the reelection of the party in power. And if six or more of the 13 keys are false, the party in power, the party holding the White House, is the predicted loser – any six or more.

Talk us through how you came to predict a Trump win.

Lichtman: Early on, the keys were inconclusive. That is, remember, six or more and the party in power is the predicted loser. And for some time, there were five keys out against the incumbent Democrats.

And since that time, that sixth key has turned against the Democrats, and that is the third party key, and that is based on an assessment that you would expect the third party candidate, in this case the Libertarian candidate, Gary Johnson, to get five per cent or more of the vote. That's a big sign of discontent with the party holding the White House. And so, again on the knife edge, you had exactly six fatal keys against the incumbent Democrats.

And remember, this was before the sex tape – this was before any of those allegations or other things emerged.

A lot of people would look at the events of the last month and say, this has got to affect the keys somehow.

Lichtman: Donald Trump's severe and unprecedented problems bragging about sexual assault and then having 10 or more women coming out and saying, "Yes, that's exactly what you did" – this is without precedent. But it didn't change a key.

By the narrowest of possible margins, the keys still point to a Trump victory. However, there are two major qualifications. And I'm not a hedger, and I've never qualified before in 30 years of predictions.

Qualification number one: It takes six keys to count the party in power out, and they have exactly six keys. And one key could still flip, as I recognised last time – the third party key, that requires Gary Johnson to get at least five per cent of the popular vote. He could slip below that, which would shift the prediction.

The second qualification is Donald Trump. We have never seen someone who is broadly regarded as a history-shattering, precedent-making, dangerous candidate who could change the patterns of history that have prevailed since the election of Abraham Lincoln.

The Washington Post, first appeared SMH