Until very recently indeed, the idea that the president of the United States might stand outside the White House and call on Communist China to investigate one of his presidential challengers would not merely have seemed far-fetched. It would also have seemed unpatriotic (presidents don’t involve foreign powers in domestic politics), unprincipled (this is the same China with which he is fighting a trade war and which may soon crack down on Hong Kong democracy protests), illegal (US law bans attempts to solicit foreign assistance to fight American elections), and a breach of his oath of office (in which he promises to protect and defend the constitution). It short, such a thing was unthinkable.

It is a mark of Donald Trump’s ability to trash the rules of domestic and international politics, and make up an entire new set of his own, that the unthinkable happened this week without causing much more than a weary collective shake of the American head. Speaking on live television outside the White House on Thursday, Mr Trump openly solicited America’s greatest international rival to help him get re-elected. Next year, Mr Trump may face a presidential contest against Joe Biden, the former vice-president, whose son Hunter – like Donald Trump Jr, as it happens – is a businessman and lobbyist with overseas interests. This week Mr Trump said: “China should start an investigation into the Bidens.”

By all formerly prevailing standards this was an extraordinary act. But it did not come out of the blue. The call to China was simply a much more public, more brazen and more reckless iteration of the very same thing that Mr Trump stands accused of doing in Ukraine, on the basis of which he is now being investigated for possible impeachment by the House of Representatives. In July, Mr Trump used a telephone call to the newly elected Ukrainian president, Volodymyr Zelenskiy, to press for investigations to be launched into the Bidens. If this did not happen, Mr Trump warned, the US would withhold military aid, a threat that was withdrawn after it became public. This week, however, evidence was produced on Capitol Hill that Mr Trump said he would only permit a White House meeting with Mr Zelenskiy if he publicly began a probe into the Bidens. Soberingly, Ukraine has now done just that.

The shredding of norms does not get more brazen than this. But the Ukraine and China cases are not freak exceptions from an otherwise ethical record. Only this week it was claimed that Mr Trump had used his first phone call to Britain’s new prime minister, Boris Johnson, on 26 July to ask the UK to provide evidence that might help undermine Robert Mueller’s investigation into Russian influence in Mr Trump’s 2016 election campaign. Similar White House demands have also been made to members of the Australian and the Italian governments.

All these efforts are egregious. For a US president to press foreign governments to investigate his rivals is not just unusual, unfair or illegal. It is also lethally dangerous to national standing and international law. If there is evidence of unethical or corrupt activity in a foreign country, it should be properly investigated by the legitimate authorities and resolved within established and accountable bilateral processes. It should not be pursued as Mr Trump did with Ukraine, demanding “a favour” in return for aid or access. It should not be multiplied, as Mr Trump has now done with China, by smearing an opponent without evidence. Such acts send a wholly lawless signal to the world, that White House favours can be won by providing dirt on its enemies and that the president is in the market for dodgy dealings with despots.

Mr Trump’s actions in Ukraine were a self-serving abuse of power. His Chinese claims are a reckless attempt to normalise his deviancy. Britain, for one, must have no part in it. Mr Trump’s actions are fuelled by refusal to accept responsibility and by fear of defeat. But he must be accountable. He must answer for his misuse of office. The political problem is that he may get away with it. The more he is criticised and implicated, the more his supporters back him. This cannot be ignored. But nor can the evidence. Mr Trump is in the dock. So are America’s political system and its standing in the world.