Trump was impeached on December 18 for abuse of power related to his dealings with Ukraine and obstruction of Congress for refusing to participate in the impeachment investigation.
House Democrats accuse the president of abusing his power in office by orchestrating a pressure campaign to get Ukraine to investigate former Vice President Joe Biden, a leading Democratic political rival, as well as launch a probe into a debunked conspiracy theory that Ukraine, not Russia, meddled in the 2016 presidential elections.
The House impeachment investigation centred on a whistle-blower complaint that detailed concerns about a phone call in which Trump asked Ukrainian President Volodymyr Zelenskyy for the investigation into Biden and his son, Hunter, who had served on the board of a Ukrainian gas company.
At the time of the call, the Trump administration was withholding nearly $400m in congressionally approved military assistance to Ukraine.
Senate Democrats want to hear from at least four witnesses in the Senate trial – a demand Republicans have criticised. For now, the question of witnesses will be discussed after opening arguments and the question-answer session.