The trigger for this fusillade is - has always been - Barack Obama. The Fox News item that Trump quoted had a clip of Obama and President Cyril Ramaphosa in a jovial mood at the Nelson Mandela lecture in Johannesburg last month.

Obama's speech, his first of consequence since leaving office, was seen as a denunciation of Trumpism, without mentioning his successor by name.

He gave a ringing endorsement of a free press - which Trump has dubbed the enemy of the people - as the bedrock of any democracy, and slammed the "utter loss of shame among political leaders when they're caught in a lie and they just double down and lie some more" - an allusion to Trump's predilection for falsehoods. "People just make stuff up," Obama said to raucous laughter.

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