Washington: US President Donald Trump has been in the White House barely four months, but he's never stopped pushing the boundaries of presidential behaviour.

Whether it's his tweets attacking lawmakers and other public officials, his misstatements and untruths, or his alienation of America's longstanding allies, he has taken a delight, it seems, in playing the bull in the china shop.

But in wading into a criminal investigation into his own campaign with his firing of FBI Director James Comey on Tuesday night, Trump has taken the kind of steps that would be routine for the dictatorial leaders - the Putins, the Dutertes, the Erdogans of the world - who he appears to admire.

Trump's decision to sack Comey just 24 hours after his former top national security aide, Michael Flynn - and, by extension, his own diligence concerning Flynn's behaviour - was publicly put through the wringer raises a question: was this political payback?