How did it come to this? The presidential election debates should represent United States democracy at its finest. Instead, the second Clinton-Trump debate centred on sordid allegations of sexual assault, threats, lies and mutual contempt.
At one stage, Mr Trump boasted that Mrs Clinton would "be in jail" if he were in charge of the legal system. Political rivals to the president get imprisoned in Mr Robert Mugabe's Zimbabwe. America is meant to live by different standards.
Sunday night's spectacle is not just embarrassing for the US. America is widely regarded as the "leader of the free world". So the rise of Mr Trump threatens to damage the prestige of democracy everywhere.
The damage is not restricted to the world of ideas. Authoritarianism and anti-Americanism are on the march, led by increasingly confident governments in Beijing and Moscow. A strong and impressive US should be central to rallying the response of the world's democracies. Instead, we had the depressing and degrading spectacle of the second Trump-Clinton debate.
Even in their current sorry state, the presidential debates have shown some of the drama and energy that distinguish US politics. Millions of people around the world watched and discussed the confrontation. The next session of China's National People's Congress will not attract a similar audience.
It is also true that neither Mr Xi Jinping of China nor Mr Vladimir Putin of Russia would ever be subjected to the kind of brutal interrogation to which American politicians are subjected on a routine basis. Instead, last week, the Russian Duma sent Mr Putin 450 roses to mark his birthday.
Yet, even so, the second presidential election debate was a desperately poor advertisement for US democracy. In some respects, Mr Trump has actually introduced some of the malign features of Russian and Chinese politics into the US. One of the strengths of the Western democratic system is that a free press and open debate are meant to expose falsehoods. Yet Mr Trump sprays out lies like a skunk trying to repel its enemies. His method seems to be to create such confusion that the truth simply gets buried amid a mass of falsehoods. This is characteristic of the current Russian propaganda system described in an aptly titled book by Peter Pomerantsev: Nothing Is True And Everything Is Possible.
The Chinese challenge to America's democratic ideology is more subtle and perhaps more dangerous because China, unlike Russia, can make a good claim to be a well-governed country. China is the largest economy in the world measured by purchasing power parity. The Chinese argue that their system selects leaders on merit, after decades of rigorous assessment. President Xi only made it to the pinnacle of state power after many years of work in the provinces and in different government jobs. He has been judged by his peers, not the voters, to be qualified to run the country.
The Chinese do not yet argue that their system should be applied around the world. But they do increasingly condemn - as agents of America, seeking to "sow chaos" - those who make the case for a more liberal political system within the Sinic world, for example in Hong Kong or Taiwan. Beleaguered liberals in Russia or China need a well-functioning US democracy as a support and an inspiration. Instead, they see a system that produces Mr Trump, a man whose political style owes more to President Putin than to President Barack Obama.
In Beijing recently, I was told that many Chinese officials quite like the idea of Mr Trump as US president "because he makes America look so bad". By contrast, US allies around the world would be dismayed to see the Oval Office occupied by an erratic "America First" narcissist like Mr Trump.
Of course, US politics has thrown up villains and melodrama before. The first great US political scandal that I followed as a child was Watergate - which also featured a "bad guy" making scandalous remarks on a secret recording. The Watergate tapes introduced the American public to the phrase "expletive deleted". Many Americans were scandalised by the profanity and cynicism of their president Richard Nixon. But the way that the US system - the courts, the press and the Congress working together - dealt with Mr Nixon was ultimately very impressive. And for all his flaws, no one doubted that Mr Nixon had the experience and the intelligence to be president.
By contrast, Mr Trump is manifestly unqualified and has thrown the US system into confusion, leaving the press and the Republican Party floundering. The fact that more than 40 per cent of Americans, and a majority of whites, are probably going to vote for him suggests that the US is in deep trouble. We can, by now, all list the ingredients that have helped create this sickness - economic stagnation, inequality, illegal immigration, the rise of social media - but the outcome threatens the prestige of democracy worldwide.
If Mrs Clinton makes it to the White House there will be relief across the West and a certain disappointment in Moscow and, perhaps, Beijing. But it will be very hard to erase the memory of this campaign. It has presented an image of a troubled, divided and deluded US to the rest of the world. As a result, it has already dealt a serious blow to the prestige and power of the West.
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