Republican and Democratic leaders in the US Senate have clashed over rules of President Trump's impeachment trial.
Democrats are demanding assurances that witnesses and documents will be allowed, to enable what they term a fair trial.
Top Democrat Chuck Schumer says the recent release of an "explosive" email about aid to Ukraine is a reminder of why openness is necessary.
Republican leader Mitch McConnell says he has not ruled out witnesses.
But he stopped short of agreeing ahead of time to take testimony during the trial.
President Trump was formally impeached by the House last week for abuse of power and obstruction of Congress.