The Senate trial of President Donald Trump is due to get underway in earnest at 1 pm (1800 GMT) on Tuesday, only the third impeachment trial of a president in U.S. history.
The 100 members of the upper chamber will gather for what is likely to be the first full day of proceedings in the case.
Last week, senators and the chief justice were sworn in.
Since then, the Democratic Party lawmakers who will head the prosecution, known as the House managers, have filed their legal brief and Trump’s defence team has done the same.
The president is accused firstly of having abused the power of his office to pressure Ukraine into announcing an investigation of his domestic political rival, Joe Biden, in order to potentially help Trump’s re-election campaign.
The second article of impeachment says he obstructed Congress’ investigation of the Ukraine affair.
Both articles were approved last month by the House of Representatives.
The president denies wrongdoing.
His legal team said on Monday that the president “is the victim” of a “rigged process” motivated by politics and that he has done nothing wrong.
The brief argues that the case is “flimsy” and that the two articles of impeachment “allege no crime or violation of law whatsoever”.
Later on, Monday Republicans proposed 12-hour daily sessions for the trial, setting a gruelling pace for the start of the case.
Democrats immediately expressed their anger, calling the proposal “a national disgrace”.
Trump is framing the impeachment as an attempt to overturn the 2016 election.
Democrats say the president’s behaviour is threatening the integrity of the election later this year.
Senators must remain silent during proceedings, which are due to take place every day except on Sundays until a decision is reached.
Two-thirds of members are required to remove a president from office.
The Senate is controlled by Trump’s Republican party and the most likely outcome is an acquittal.