The Trump administration’s story of why FBI director James Comey was fired, which began to twist almost as soon as it was told, took another turn on Thursday when the president said this to NBC News:

“And in fact, when I decided to just do it, I said to myself, I said, ‘You know, this Russia thing with Trump and Russia is a made-up story’.”

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Mr Trump was well within his legal rights in sacking Mr Comey. “The president can fire an FBI director for turning up in the wrong tie or socks, if he wants to,” said Stanford Law School professor Pamela Karlan.

“The real question is whether in doing so he was attempting to impede the FBI’s investigation.”

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Under the constitution, impeachment proceedings require a president to have committed “treason, bribery, or other high crimes and misdemeanours”. Ruling out the first two, in this case, the terms “high crimes and misdemeanours” are loosely defined but obstruction of justice, if proved, would “clearly count”, Ms Karlan said.

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Not yet, said Ms Karlan. “Right now this is a president behaving extraordinarily badly. The question is what will the various institutions of government do about this.

“If it becomes clear that the president is trying to obstruct justice and Congress does nothing, that moves us towards a constitutional crisis. If Congress doesn’t fulfil its role as a check on the president, that’s a real problem.”