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.
He is the third president in US history to be impeached. However, he is unlikely to be removed from office, as his Republican party has a majority in the Senate, where the trial will be held as stipulated in the US Constitution.
The trial is expected to begin next month, after the holiday break.
But Democrats have so far refused to hand over the articles of impeachment voted through in the House, the charges, to the Senate.
They want assurances from Mr McConnell that their chosen witnesses, at least four current and former White House aides with knowledge of the Ukraine affair, will be allowed to testify.
Mr Trump is accused of pressuring Ukraine’s president to start an investigation into his political rival, Democratic presidential front runner, Joe Biden.
Mr Trump is accused of doing this by withholding military aid and making a White House visit contingent on co-operation.