The curtain came up on day two of the House of Representatives' public impeachment hearings and, once again, the proceedings started off with a bang.

On Wednesday, it was disclosure of an  dreamgame overheard phone call between Donald Trump and US Ambassador to the EU Gordon Sondland during which the president may have asked about Ukrainian investigations.

Friday's big developments included the White House release of a contradictory readout of the Mr Trump's first phone call with Ukrainian President Volodymy Zelensky and a presidential tweet that had Republicans in Congress scrambling.

Mr Trump has boasted that his conduct while in office - blunt language and shoot-from-the-hip tweeting - is "modern-day presidential". If so, welcome to a modern-day presidential impeachment hearing.

Just over an hour into Marie Yovanovitch's testimony, Mr Trump launched the kind of Twitter fusillade that has become a regular part of his political repertoire. He questioned the ambassador's competence, noted that the Ukrainian president spoke unfavourably about her and pointed out that he has the right to fire diplomats at will.

What makes the moment historic is that Committee Chair Adam Schiff gave Yovanovitch a chance to refute the president's tweet almost in real time.

Democrats are already characterising the president's behaviour as witness intimidation - and the latest attack by the president against one of his แทงบอลสด own government employees.

Republicans, whose reported strategy was to avoid directly impugning the reputation of a long-serving, well respected diplomat, will once again find a president who has changed the rules of engagement on the fly.