Democrats have pressed their case for removing US President Donald Trump from office by using the words of his own allies against him at his impeachment trial.
The Democratic House of Representatives lawmakers serving as prosecutors in the Senate trial presented the second of their three days of opening arguments on Thursday.
They appealed to senators to convict Trump on two charges - abuse of power and obstruction of Congress.
The US Constitution sets out the impeachment process for removing a president who commits "high crimes and misdemeanors".
Trump's legal team has argued that the House charges were invalid because impeachable offences must represent a specific violation of criminal law.
"Impeachment is not a punishment for crimes," Democratic Representative Jerrold Nadler told the assembled senators.
"Impeachment exists to address threats to the political system, applies only to political officials and responds not by imprisonment or fines but only by stripping political power."
Nadler played a video clip of one of Trump's most prominent defenders, Republican Senator Lindsey Graham, arguing during the 1999 impeachment trial of President Bill Clinton that presidents could be impeached even if the conduct in question was not a statutory criminal violation.
Nadler also played a 1998 video clip of Alan Dershowitz, a member of Trump's legal team, recognising abuse of power as impeachable.
He also cited a memo written by Attorney General William Barr, a Trump appointee, that made the same point.
Trump's legal team has stated that abuse of power is a "made-up theory" for an impeachable offence "that would permanently weaken the presidency by effectively permitting impeachments based merely on policy disagreements".
The charges against Trump arise from his request last year that Ukraine investigate Democratic rival Joe Biden and the president's actions to impede a House inquiry into the matter.
"His conduct is not America first. It is Donald Trump first," Nadler said.
"As our recital of the facts indicated, the articles are overwhelmingly supported by the evidence amassed by the House, notwithstanding the president's complete stonewalling, his attempt to block all witnesses and all documents from the United States Congress," he said.
Trump is almost certain to be acquitted by the 100-member Senate, which has 53 Republican members. A two-thirds majority is needed to remove him from office.
The case focuses on Trump's request that Ukraine investigate Biden, a top contender for the Democratic nomination to face Trump in the November 3 presidential election, and Biden's son on unsubstantiated corruption allegations.
Trump also asked Ukrainian President Volodymyr Zelenskiy to investigate a discredited theory beneficial to Russia that Ukraine worked with Democrats to hurt Trump in the 2016 US election.
Last year, Trump temporarily withheld $US391 million ($A572 million) in military aid to Ukraine, which Democrats say was leverage for his demands.
Trump denies wrongdoing.