What will it take for a former president to speak out against Trump?

The silence from former occupants of the Oval Office has been deafening as the incumbent cuts a destructive path

The stadium announcer called on the crowd to give a warm welcome to a very special guest". A cheer went up as basketball fans realised that Barack Obama was in their midst. The former US president rose to his feet, smiled and waved before watching the Los Angeles Clippers take on the Detroit Pistons on Wednesday night.

It was a jarringly normal scene at a profoundly abnormal time. The previous evening, Donald Trump had delivered the longest ever presidential address to Congress, a dark, divisive tirade strewn with lies and insults - he called Joe Biden the worst president in American history" and Senator Elizabeth Warren Pocahontas".