US House of Representatives Speaker Nancy Pelosi has warned Republicans not to become "the president's henchmen" as the Trump impeachment trial began, APA reports citing BBC.
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."