US House of Representatives Speaker Nancy Pelosi has warned Republicans not to become “the president’s henchmen” as the Trump impeachment trial began.
Mrs Pelosi renewed her call for fresh evidence and witnesses to be admitted in the forthcoming hearings.
Pre-trial proceedings have begun with the charges being formally read aloud to the Senate.
Donald Trump is the third US president ever to be impeached, but the previous two were not removed from office.
He is accused of abuse of power and obstruction of Congress, and brands the case against him as a politically motivated “hoax”.
Speaking at her weekly press conference on Thursday, Mrs Pelosi said: “I hope that the senators do not become part of the president’s henchmen.”
She spoke shortly before the trial of the president got underway in the US Senate, which is controlled by the president’s fellow Republicans.
The articles of impeachment were read out on the floor of the chamber by lead prosecutor Adam Schiff.
The Democratic congressman said no president had ever sought to impede an impeachment investigation so thoroughly.
Supreme Court Chief Justice John Roberts will be sworn in as presiding judge before the senators take their oaths as jurors.