– Congress headed toward a long standoff over the parameters of the Senate trial of President Donald Trump as all sides dug in Monday, with Democrats demanding documents and witnesses while Republicans mocked the House's delay in transmitting the impeachment articles across the Capitol.

Through media appearances, letters and tweets, Trump and Senate Majority Leader Mitch McConnell, R-Ky., chided House Speaker Nancy Pelosi, D-Calif., calling her decision to hold on to the impeachment articles "unfair" and "absurd," respectively. Senate Minority Leader Chuck Schumer, D-N.Y., meanwhile, accused White House officials of withholding "highly relevant" documents.

With Congress not slated to return to Washington until Jan. 6, senior officials prepared for a period of several weeks without any resolution.

"We'll find out when we come back in session where we are," McConnell told reporters in Louisville.

McConnell also referred Kentucky reporters to comments he made earlier Monday on Fox News's "Fox & Friends," accusing Pelosi of holding an "absurd position," and said she seemed to be trying to tell his chamber how to run a trial.

Since Wednesday's near-party-line House vote to impeach Trump, Pelosi has declined to formally send the two articles across the Capitol, the first necessary step to begin the trial.

Pelosi wrote in a tweet Monday that the House cannot move forward with choosing impeachment managers for the Senate trial "until we know what sort of trial the Senate will conduct."

"President Trump blocked his own witnesses and documents from the House, and from the American people, on phony complaints about the House process," Pelosi tweeted. "What is his excuse now?"

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 Zelensky 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 Kiev conspired with Democrats to interfere in the 2016 presidential election.

While the articles approved last week relate only to Ukraine, Democratic lawyers for the House Judiciary Committee signaled Monday that they are willing to consider additional charges against Trump related to the two-year investigation by a special counsel probing Russia's intervention in the 2016 elections.

The lawyers filed a motion in federal court asking a judge to compel the testimony of former White House counsel Donald McGahn because the Judiciary Committee is "continuing to conduct its inquiry into whether the President committed other impeachable offenses."

Schumer pressed his case in a letter to Senate colleagues Monday that new documentary evidence needs to be part of the impeachment trial.

In his letter, he said that the House had amassed "a tremendous amount of evidence" in support of impeaching Trump but noted that the president 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, $391 million in military assistance to Ukraine."

What happens next in the impeachment of President Trump?

Aware for several weeks that impeachment was headed their way, McConnell and Schumer did not hold a discussion until Thursday, the day after the House impeached Trump. The two senators stuck to their positions and emerged with no agreement.

On the Senate floor later that day, McConnell declared they were at an "impasse," and the chamber shuttered for a 2½-week break over the holidays.

McConnell signaled during the Louisville news conference Monday that he doesn't expect much progress on the impasse before lawmakers return to Washington after the holidays.

When a 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."