No matter who triumphs on Tuesday, the US election has been a big win for China's national propaganda machine, which has gleefully catalogued the seemingly endless parade of skeletons marching out of America's political closet.

For decades Beijing has disparaged US democracy, calling into question the most basic building blocks of the state, from competitive elections to freedom of the press.

Ahead of this year's election, Chinese journalists received instructions to write stories making American politics look bad, according to sources familiar with the orders.

As the primaries began, one reporter at a major state-run media organisation, who spoke to AFP on the condition of anonymity, worried he would have a hard time finding sufficiently damning material.

His doubts were unjustified.

Even America's fiercest defenders agree the contest between Donald Trump and Hillary Clinton has shown democracy at its ugliest.

And China's media have happily joined in.

"The innumerable scandals, rumours, conspiracy theories and obscenities make it impossible for a person to look away," the official Xinhua news service wrote in a commentary last week, likening the 2016 election to a train wreck.

A commentary in the online edition of the Communist Party mouthpiece the People's Daily said the circus-like debate series between the two candidates "clearly shows the decline" of the US political system.

But it did not need to make the argument itself; it simply used a long list of quotes from prominent American media outlets across the political spectrum: "tacky", "ugly", "ear-splitting".

"No matter who gets the job, the memory of the presidential election will live on," the nationalistic Global Times, which has close ties to the party, wrote with relish on Thursday.

- Trump windfall -

Beijing has long aimed its propaganda barrels at the US, but the government has stepped up its anti-Western commentary under President Xi Jinping.

A leaked political memo from 2013, known as Document Number 9, listed "Western constitutional democracy" and its values, including media independence, as top threats to the Chinese Communist Party's rule.

Chinese cadres subsequently mounted a campaign denouncing the influence of so-called "hostile foreign forces" on a wide cross-section of society, from universities to newspapers, criticising the ideological agenda being pushed by the US as hypocritical and dangerous.

But China's propaganda bosses are unlikely ever to have imagined that a US presidential candidate would be doing their work for them.

Donald Trump's insistent criticism of the US political system as "rigged" has been a windfall for Chinese commentators.

His regular assaults on American media as biased and corrupt could have been lifted from an October Global Times op-ed that claimed the country's news coverage was universally pro-Clinton.

"Subjectivity has long dominated US media, but this is laid bare by this year's election," the paper said.

By extension, the election reporting called into question Western outlets' "negative" coverage of China, it asserted.

"They elaborated dissidents' stories while neglecting China's progress in human rights conditions," the paper said.

But China media commentator Jeremy Goldkorn pointed out that it was "a difficult balancing act for Chinese propagandists".

"Even if one of the candidates is a clown, it does not escape Chinese people that American citizens have an admirable involvement in the way their country is run," he told AFP.

- 'Debased' -

There is no clear consensus among Chinese election observers on which candidate would be easier to work with.

Many believe Clinton, in the light of her tenure as secretary of state, is more likely to pursue an assertive US posture on such issues as human rights and military affairs, while Trump's protectionist views might be harmful for trade.

"For China, they both have advantages and disadvantages," Xu Tiebing, an expert on US politics at the Communication University of China, told AFP.

Clinton is closer to a "traditional politician", he said, making her more predictable, while Trump's "thinking is harder to anticipate".

But one thing was clear, he said: "The American presidential election has been debased to a high degree."