WASHINGTON (WASHINGTON POST) - Both sides dug in on Monday (Dec 23) in the impasse over a Senate trial of President Donald Trump, with Republican Senate Majority Leader Mitch McConnell chiding Democratic House Speaker Nancy Pelosi for the delay in transmitting articles of impeachment, a position he called "absurd".

Pelosi, meanwhile, insisted that before moving forward, Democrats need to know "what sort of trial the Senate will conduct.

Democrats are seeking to use newly released emails showing that the White House put a hold on military aid to Ukraine less than two hours after Trump's July call with the country's leader to bolster their case that a Senate trial should include witnesses.

At the heart of the Democrats' case is the allegation that Trump tried to leverage a White House meeting and military aid, sought by Ukraine to combat Russian military aggression, to pressure Ukrainian President Volodymyr Zelenskiy to launch an investigation of former vice-president Joe Biden and his son Hunter Biden, as well as a probe of an unfounded theory that Kyiv conspired with Democrats to interfere in the 2016 presidential election.

McConnell signalled during a news conference that he does not expect much progress on the impasse over a Senate trial before lawmakers return to Washington after the holidays.

"We'll find out when we come back in session where we are," the Senate majority leader told reporters in Louisville.

McConnell referred them to comments he made earlier on Monday on Fox News' Fox & Friends.

During that interview, he accused Pelosi of holding "an absurd position" and said she seems to be trying to tell his chamber how to run a trial.

"I don't have anything to add," McConnell told reporters in Louisville.

When another reporter asked about Pelosi holding onto the articles of impeachment, McConnell said, "As I've said repeatedly, we can't take up a matter we don't have."

Asked what role impeachment might play in his bid for reelection next year, McConnell replied, "Who knows?"

The first Senate votes of the new year are scheduled Jan 6.

Meanwhile, Democratic Senate Minority Leader Chuck Schumer pressed his case in a letter to Senate colleagues on Monday that new documentary evidence needs to be part of an impeachment trial.

In his letter, Schumer said that the House had amassed "a tremendous amount of evidence" in support of impeaching Trump, but noted that Trump had directed his administration to defy subpoenas for documentary evidence.

"As a result of this directive, the White House, Department of State, Office of Management and Budget and other agencies refused to produce a single document in response to the House's duly-issued subpoenas," Schumer wrote.

He said the documents the Senate should seek fall into three categories: "(1) the effort to induce and pressure Ukraine to announce certain political investigations; (2) the withholding of a White House meeting desperately sought by the newly elected President of Ukraine; and (3) the order to hold, and later release, US$391 million (S$530 million) in military assistance to Ukraine."

"Production of these documents for the Senate would... ensure fundamental fairness and transparency, since the President could otherwise seek to selectively introduce documents before or during the trial in a manner that Senators could not independently evaluate as credible or reliable," Schumer wrote.