Opening arguments will begin in Donald Tump's impeachment trial after an emotional first day ended with the Senate voting to hear the case for convicting the former president of inciting the riot at the US Capitol even though he is no longer in office.

House Democrats prosecuting the case and the former president's lawyers will lay out their opposing arguments before the senators, who are serving as jurors, on Wednesday.

The defence lost the vote seeking to halt the trial on constitutional grounds, 56 to 44, leaving Mr Trump fuming over his lawyers' performance and allies questioning the defence strategy. Some called for yet another shakeup of his legal team.

House prosecutors on Tuesday wrenched senators and the nation back to the deadly attack on Congress, showing a graphic video of the January 6 mob violence that stunned the world as hundreds of rioters ransacked the building to try to stop the certification of Democrat Joe Biden's victory. Five people died.

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That detailed and emotional presentation by Democrats was followed by meandering and occasionally confrontational arguments from the Trump team, which insisted that his remarks were protected by the First Amendment and asserted that he could not be convicted as a former president.

Senators, many of whom fled for safety themselves on the day of the attack, watched and listened, unable to avoid the jarring video of Trump supporters battling past police to storm the halls, Trump flags waving.

More video is expected on Wednesday, including some that has not been seen before.

The heavy emotional weight of the trial punctuates Mr Trump's enduring legacy as the first president to face impeachment trial after leaving office and the first to be impeached twice.

While many minds are made up, the senators will face their own moment to decide whether to convict or acquit Mr Trump of the sole charge of "incitement of insurrection".

Maryland Democrat Jamie Raskin declared in opening remarks: "That's a high crime and misdemeanour. If that's not an impeachable offence, then there's no such thing."

Mr Trump's lawyers insist he is not guilty, his fiery words just figures of speech.
Security remained extremely tight at the Capitol, a changed place after the attack, fenced off with razor wire and with armed National Guard troops on patrol. The nine House managers walked across the shuttered building to prosecute the case before the Senate.

White House press secretary Jen Psaki said Mr Biden would not be watching the trial of his predecessor.

"Joe Biden is the president, he's not a pundit, he's not going to opine on back and forth arguments," she said.

With senators sworn to deliver impartial justice, the trial started with the Democratic House managers' gripping recollections, as they described police officers maimed in the chaos and rioters parading in the very chamber where the trial was being held.

Mr Trump's team countered that the constitution did not allow impeachment at this late date. Though the trial now proceeds, that is a legal issue that could resonate with Republicans eager to acquit Mr Trump without being seen as condoning his behaviour.

Lead defence lawyer Bruce Castor said he shifted his planned approach after hearing the prosecutors' opening and instead spoke conversationally to the senators, saying Mr Trump's team would do nothing but denounce the "repugnant" attack and "in the strongest possible way denounce the rioters".

He appealed to the senators as "patriots first", and encouraged them to be "cool headed" as they assessed the arguments.

Mr Trump's lawyer David Schoen turned the trial towards starkly partisan tones, saying the Democrats were fuelled by a "base hatred" of the former president.

The early defence struggles also underscored the uphill battle that Mr Trump's lawyers face in defending conduct that preceded an insurrection that senators themselves personally experienced.

Although they will almost certainly win Mr Trump's acquittal - by virtue of the composition of the Senate - they nonetheless face a challenge of lowering the emotion from a trial centred on events that remain raw and visceral, even for Republicans.

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Republicans made it clear that they were unhappy with Mr Trump's defence, many of them saying they did not understand where it was going - particularly Mr Castor's opening.

While the 56 to 44 vote affirmed the Senate's authority under the constitution to decide the case even after the president had left office, the total was still far from the two-thirds threshold of 67 votes that would be needed for conviction.

The trial is expected to continue into the weekend.