IF the circus must come to town, what better place than Vegas? Tonight’s live US presidential debate comes, in fact, from the University of Nevada campus in Las Vegas rather than one of the glitzier entertainment palaces on the Strip.

There is, nevertheless, something fitting in this bout taking place in the postcode that has played host to Elvis Presley, Frank Sinatra and Judy Garland, not to mention Siegfried & Roy and their white tigers. As Sinatra sang, the shark has pretty teeth, dear, as did those tigers, but wait till you see the fangs bared when Donald Trump and Hillary Clinton go head-to-head for the final time on live television.

Tensions were high for the second debate on October 9 and the tone of the campaign has since deteriorated, with a number of women coming forward to allege that they were touched or otherwise treated inappropriately by the Republican nominee.

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He has denied this, calling the claims “lies” manufactured by the Clinton camp and peddled by an acquiescent media. There possibly could not be a worst time for the two candidates to make their final pitches on policy in a sober, illuminating way but, as has been the case so often with this contest, the electorate has to take what it is given. Policy? That went out of the window months ago. This election is being played out as high-octane, low-brow entertainment.

As for how the televised debate will affect the polls, if past experience is any guide, there will be a brief bump for Mrs Clinton before the numbers settle down again, though settling down is hardly an apt phrase to use. The polls are all over the place, with some giving Mrs Clinton a double-digit lead, others suggesting it is a single figure and even Mr Trump has been able to claim he is winning in some surveys.

If the polls turn out to be wildly wrong once more, as they have been in UK contests of late, it is difficult to see the survey industry surviving this one. But if you absolutely had to put money on an outcome, many believe that Mrs Clinton will win, if only because the electoral college arithmetic is on her side. This leaves America with a question to answer, one which has not hitherto been given the attention it deserves. Instead of worrying what a Trump presidency would be like, how will America deal with a Trump defeat?

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For those of a nervous disposition, the noises coming from the Trump camp in the past week have been particularly troubling. The narrative is simple: the fix is in and the electorate is once again being played for chumps by the elite (says the millionaire businessman and his chums). It is not just Mrs Clinton and her media backers who are rigging the election, Mr Trump alleges. He has gone as far as saying the polling places in some states are not to be trusted.

This goes beyond normal electoral jousting to something more sinister. If a candidate throws into doubt the fairness of an election before the event it stands to reason that the result will be questioned and held in contempt later. America does not have to go too far into its past for an example of the last time that happened. Remember Bush versus Gore in 2000, the hanging chads, the recounts, the battle in the Supreme Court over the result? Looked at from over here, a strange madness seemed to grip America in those days: neighbours turned against each other, friends fell out, red state rallied against blue state and hardly anyone had a good word to say about Florida. One of the world’s greatest democracies lost face, lost confidence and, for a time, lost its head.

There is less than a month until the election. If the Trump campaign is getting hot and bothered about the poll at present, what will the temperature of his supporters be come November 8, and how will that manifest itself? Regardless of what you think about Mr Trump and his policies, it is undeniable that some of the things he has said chime with the views of a lot of Americans, some of whom will be heading to the voting booths for the first time in many years.

No-one knows exactly how many Trump supporters there are (remember, as with all polls, nobody knows anything) but we are not talking one man and his dog here. In a poll for Politico/Morning Consult, 73 per cent of Republicans surveyed reckoned the election could be “stolen” from Mr Trump. If those supporters believe what he says, that their votes are not worth the paper they are punched on, are they likely to shrug their shoulders and think better luck next time? Judging by the vox pops in the Trump rallies, no.

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There is another way a Trump defeat could play out other than in ugly scenes at polling booths and afterwards. In this scenario, Mr Trump’s candidacy is seen as one of the greatest marketing campaigns of modern times in which the product being punted is the man and his various enterprises.

He never thought he could win, goes the theory, but he wants to ride the Trump train as far as he can in the hope the exposure ultimately adds even more millions to his bank account.

Those who believe this will have read with interest reports in the Financial Times this week saying the candidate’s son-in-law, Jared Kushner, has been floating the idea of setting up “Trump TV”.

Mr Trump has previously said he has no interest in setting up a media company and neither of the parties mentioned in the FT story would comment. Just imagine, though, all those disaffected voters becoming engaged consumers, their rage neatly channelled into viewing, listening and subscription figures for a Trump network.

Setting up such an operation in a crowded market (Fox News would have something to say about it, for a start) would be difficult and expensive – TV stations are up there with English football clubs as money pits – but if Trump did not want a dedicated outlet of his own he could always direct supporters in the direction of Breitbart News, run by Stephen K Bannon, Mr Trump’s campaign chief.

If either scenario happens, a new media empire being created by Mr Trump, or a huge expansion of an already existing right-wing outlet, might be good for media diversity, but it is going to be very bad news for Mrs Clinton.

During her husband’s presidency she was quick to blame his woes on a “vast right-wing conspiracy”. If Mr Trump’s media dreams, should they exist, come to pass, Mrs Clinton will be challenged every step of the way, on everything.

Those crazy times of the 1990s, when the Clintons were under siege over everything from Whitewater to Monica Lewinsky, could look like a day at the beach.

Onwards, then, to tonight’s debate. It was always the case that this contest would be ugly. With two such polarising candidates, how could it have been otherwise? The worry now is that the rot in the US political system has taken hold and can only worsen.