WASHINGTON, United States (AP) — House Democrats were preparing to outline their case for removing President Donald Trump from office in a legal brief due Saturday, as opposing sides in the impeachment case look ahead to the opening of the historic trial in the Senate.

Trump on Friday appointed several nationally known lawyers to the team that will defend him in the proceedings, set to open Tuesday afternoon.

The submission of the legal brief, due by 5:00 pm Saturday, follows the latest revelations in the case against Trump.

Democrats on Friday released more information — documents, text messages, audio and photos — turned over by Lev Parnas, an indicted associate of Trump lawyer Rudy Giuliani. The release included multiple photos of Parnas, a Soviet-born Florida businessman, posing with Giuliani or Trump or Donald Trump Jr, the president’s son.

It included messages between Parnas and a staff member for Rep Devin Nunes, R-Calif, a Trump ally who opposes the president’s impeachment by the House. Parnas appeared to be connecting the staff member to Ukrainian officials who pushed unfounded corruption allegations against former Vice President Joe Biden.

The documents also raised more questions about the surveillance and security of former Ambassador Marie Yovanovitch. In them, an unidentified individual with a Belgian country code appears to describe Yovanovitch’s movements.

The document release followed Thursday’s announcement by the Government Accountability Office that the White House violated federal law by withholding congressionally approved security aid to Ukraine, which shares a border with a hostile Russia.

In response, the White House disagreed and said it does not have to follow decisions by the accountability office because it is an arm of Congress. White House officials also have noted that Trump eventually sent the US$400 million in aid to Ukraine.

But the GAO report and Parnas documents intensified the pressure senators have been under to call more witnesses for the trial, a major source of disagreement between Democrats and Republicans that has yet to be resolved. The White House has instructed officials to disregard subpoenas from Congress seeking for them to appear as witnesses or turn over documents or other information.

Trump on Friday named Ken Starr, the prosecutor whose investigation two decades ago led to the impeachment of President Bill Clinton, along with former Harvard Law professor Alan Dershowitz, to his defence team.

The additions bring experience in the politics of impeachment as well as constitutional law to Trump’s made-for-TV legal team. Both Starr and Dershowitz have been fixtures on Fox News Channel, Trump’s preferred television network.

Dershowitz said he will deliver constitutional arguments defending Trump from allegations that he abused his power. Trump is also accused of obstructing Congress as it sought to investigate pressure he applied on Ukraine’s president to announce an investigation into Trump’s political rivals as the president withheld the security aid and a White House meeting as leverage.

Trump says he did nothing wrong and argues that Democrats have been out to get him since before he took office.

A legal brief from the White House laying out the contours of Trump’s defence was due by noon Monday, and White House attorneys and Trump’s outside legal team have been debating just how political the document should be.

Some in the administration have echoed warnings from Senate Majority Leader Mitch McConnell, R-Ky., that the pleadings must be sensitive to the Senate’s more staid traditions and leave some of the sharper rhetoric exhibited during the House proceedings to Twitter and cable news.

Dershowitz is a constitutional expert whose expansive views of presidential powers echo those of Trump. Starr is a veteran of Washington’s partisan battles after leading the investigation into Clinton’s affair with a White House intern. The House impeached Clinton, who then was acquitted at his Senate trial. Trump is expecting the same outcome from the Republican-led chamber.