Republican and Democratic leaders in the US Senate have clashed over the rules of President Trump's impeachment trial.
Democrats want assurances 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.
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.
he 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.
"We haven't ruled out witnesses," Mr McConnell told Fox News on Monday.
He suggested holding a trial similar to former President Bill Clinton's in 1999, in which senators decided which witnesses to call after opening arguments and a written question period.
Mr McConnell accused Speaker of the House Nancy Pelosi of holding "an absurd position" for delaying handing over the impeachment articles and said she is "apparently trying to tell us how to run the trial".
What are Democrats saying?
They renewed their demand for witnesses over the weekend after an email emerged suggesting the White House sought to freeze aid to Ukraine just 91 minutes after Mr Trump spoke to President Volodymyr Zelensky by phone in July. That call is at the centre of the allegations against Mr Trump - charges he denies.
Chuck Schumer said he and his Republican counterpart remain at an impasse after holding a "cordial" meeting on Thursday to discuss trial rules.