Senators voted 56-44 in favour of the constitutionality of the trial, rejecting a bid by Trump's lawyers to throw it out on grounds that a former president cannot be put on trial.
The result showed the nearly impossible task of getting the two thirds majority needed to convict Trump, who is facing impeachment for “incitement of insurrection.”
In their opening case to the Senators, sitting as jurors, Democratic Representatives argued that Trump broke his oath in a bid to retain power after he lost the November election to Joe Biden.
In an emotional presentation, they detailed the lies Trump spread about vote rigging, and his repeated pressuring of officials, including then vice president Mike Pence, to try and stop the transfer of power.
Then on 6 January Trump told supporters near the White House to "fight like hell", and crowd then attacked the Capitol building, where Congress was certifying the results of the vote, and Biden’s victory.
Senators were shown video footage from the 6 January speech and the attack on the Capitol by a mob chanting pro-Trump slogans, smashing doors and swarming police.
"If Congress were to just stand completely aside in the face of such an extraordinary crime against the Republic, it would invite future presidents to use their power without any fear of accountability," said Democratic impeachment manager Joe Neguse.
Trump’s defence team, however, said the Senate had no jurisdiction to try a president once he has left office. Laywer David Schoen warned that the impeachment threatened to "tear this country apart," leaving the US "far more divided and our standing around the world will be badly broken".
The defence also argues that what Trump said during the 6 January rally is protected by the constitutional right to free speech, and did not amount to ordering the assault on Congress.
Substantive opening arguments will begin at noon Wednesday. The trial is expected to continue into the weekend.
Trump is the first US president to be tried after leaving the White House, and the first to face two impeachment trials. He was acquitted in 2020 of abuse of power
A second acquittal is all but certain, as Republican Senators hesitate to turn against Trump who still retains large support of Republican voters.
(with wires)
