Democrats and Republicans in the United States Senate are wrangling over whether to call new witnesses and introduce additional evidence in the impeachment trial of President Donald Trump.

With Trump's allies in control of a 53-47 majority in the Senate, Democrats are framing the issue before senators as a question of whether Republicans will "cover up" Trump's actions by refusing witnesses.

Republicans are posturing that the House of Representatives impeachment investigators rushed through an unfair process and failed to lock up sufficient proof of their case.

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Meanwhile, new revelations in the still-unfolding scandal are raising new questions and prompting calls for the Senate to hear more witnesses.

The House impeachment inquiry produced 35 hours of public hearings with 12 witnesses and assembled hundreds of pages of documents for referral to the Senate.

Here is a roundup of the new evidence that has come out since the House voted to approve articles of impeachment on December 18:

Lev Parnas breaks silence

Lev Parnas, a Ukranian-American businessman, said in recent media interviews that Trump had full knowledge of the pressure campaign on Ukraine - and directed it - through his personal lawyer Rudy Giuliani and top US officials.

"Mr Parnas's public interviews in the last 24 hours shed additional insights into the origins of the scheme, the work he and Rudy Giuliani were doing on the president's behalf, and other members of the administration who were knowledgeable," Adam Schiff, the lead House manager prosecuting the case against Trump, said earlier this month.

In news interviews, including ones with Rachel Maddow on MSNBC, Parnas named Vice President Mike Pence, Secretary of State Mike Pompeo, Attorney General William Barr and former Energy Secretary Rick Perry.

Lev Parnas says Trump directed a pressure campaign against Ukraine [File: Seth Wenig/The Associated Press]

Parnas is under federal indictment by US prosecutors in New York for campaign finance violations. He is cooperating with prosecutors.

Trump says he doesn't know Parnas, but Parnas's private Instagram feed featured pictures of Parnas with Trump, Giuliani, and members of the Trump family.

Parnas provided emails and text messages with Giuliani and Ukrainian officials to House investigators - communications that appear to broadly corroborate his story.

Parnas's documents will be included in the House's referral of evidence to the Senate, and Schiff said the House may seek to call Parnas as a witness.

Parnas also alleged that Trump threatened to withhold more than just military aid.

"The message was it wasn't just military aid. It was all aid," Parnas told MSNBC. "Basically, the relationship would be sour. We would stop giving them any kind of aid."

John Bolton offers to testify

Former National Security Advisor John Bolton issued a statement on January 6 saying he would comply with a subpoena from the Senate. His lawyers said he has relevant new information.

Bolton left the White House in September after a falling-out with Trump, who suggested in a tweet that Bolton was fired, saying his "services are no longer needed". Bolton said he quit.

Former National Security Advisor John Bolton has said he will testify if subpoenaed in the Senate trial [Pablo Martinez Monsivais/The Associated Press]

Fiona Hill, the White House's former top Russia expert who worked for Bolton on the National Security Council, told the House that Bolton called the pressure campaign on Ukraine a "drug deal" that he didn't want to be part of.

Bolton declined an invitation to testify in the House impeachment inquiry. He could provide a first-hand account of Trump's actions.

Watchdog says White House broke law

On January 16, the US Government Accountability Office, a non-partisan accounting agency that reports to Congress, issued a legal finding that the Trump administration broke key federal budget laws by unilaterally withholding $391m in congressionally approved military aid to Ukraine.

"Faithful execution of the law does not permit the president to substitute his own policy priorities for those that Congress has enacted into law," the eight-page GAO opinion said.

Two White House budget officials resigned from their jobs over concerns that withholding the Ukraine aid was illegal, according to testimony in the House.

Democrats said the GAO opinion further bolsters their demand that the Senate call acting White House Chief of Staff Mick Mulvaney as a witness.

Emails show hold on Ukraine aid

A series of public releases of government emails shed new light on how the White House moved to block military aid to Ukraine while Trump was pressuring Ukrainian President Volodymyr Zelenskyy to open political investigations of former Vice President Joe Biden and his son, Hunter Biden.

Pentagon emails published by the Just Security website on January 2 showed that Trump directed the hold on Ukraine aid even as US defence officials raised serious concerns about the legality of the hold.

Reports detailed acting chief of staff Mick Mulvaney's role in halting military aid to Ukraine [File: Patrick Semansky/The Associated Press]

On December 29, The New York Times newspaper published a report - based on interviews and previously undisclosed emails and documents - on the alarm within the Trump administration over the delay of aid to Ukraine. The report included new information on Mulvaney's role and efforts by White House lawyers to justify the delay after the fact.

The Pentagon released emails on December 22 showing that 90 minutes after Trump's July 25 phone call with Zelenskyy - in which Trump asked Zelenskyy to open investigations of the Bidens - a top White House aide instructed the US Department of Defense to "hold off" on sending military aid to Ukraine.