Chilling video evidence on the first day of Donald Trump's second impeachment trial, showing his frenzied supporters smashing their way into the US Capitol, revived a national trauma.

Combined with a botched, illogical opening by his defense team, it also raised a perennial question: what exactly would it take for Republican senators to finally hold the ex-President to account?

The graphic footage, taken from multiple cellphone cameras and television feeds on January 6, showed the moment when a crowd goaded by Trump invaded Congress just at the moment when lawmakers were certifying President Joe Biden's election victory.

The horrific scenes, played by House Democratic impeachment managers, put viewers inside the raging mob. It brought them into ornate congressional chambers as rioters chanting Trump's name and "Stop the Steal" hunted lawmakers and then-Vice President Mike Pence.

The Senate trial resumes at noon on Wednesday. House impeachment managers have already established a clear line of consequence between Trump's rhetoric and the assault.

They also made a strong case to senators sitting as jurors that trying an ex-President impeached in office for effectively attempting a violent coup to stay in power was constitutional.

"If that's not an impeachable offense, there's no such thing," lead House impeachment manager Rep Jamie Raskin told a silent Senate.

It was a statement that Trump's legal team, who insulted the dead and injured from the riot by calling the mashup an example of "blood sport" whipped up by a "movie company," never answered. That's perhaps because there is no good response.

The defense response was a fair metaphor for aspects of Trump's performance in the presidency: it was unprepared, contradictory, impulsive and politicized.

Objective observers and subjective ones including Republican senators who have already made up their mind panned the opening contributions of lawyer Bruce Castor Jr. and to a lesser extent David Schoen. 

Some Republicans said the presentations by House managers were more convincing than those in Trump's first trial last year after he was impeached for trying to get Ukraine to damage Biden. 

Far from helping the ex-President, his team, called in at the last minute because his previous representatives pulled out, appear to have confused senators.

"I couldn't figure out where he was going. (He spent) 45 minutes going somewhere," said Alaska Sen. Lisa Murkowski, one of six Republicans who voted with Democrats in the Senate to declare the trial constitutional.

"What the hell is going on?" one adviser to Trump simply asked.