Senate Democrats are considering offering Republicans a deal during President Donald Trump's impeachment trial, according to The Washington Post.

They are considering proposing that Hunter Biden testify in exchange for testimony from a key administration official, such as the former White House national security adviser John Bolton, The Post said.

Trump's obsession with getting Ukraine to announce an investigation into Biden — whose father, Joe, is a 2020 Democratic presidential frontrunner — is at the heart of his impeachment.

Democrats and Republicans are haggling over the terms of the trial, with Democrats pushing for new witnesses to be heard and Republicans resisting.

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Senate Democrats are privately considering offering Republicans a witness-swap deal in President Donald Trump's impeachment trial, The Washington Post reported Tuesday night.

According to the report, Democrats are considering proposing that Hunter Biden take the stand in exchange for testimony from a key administration official such as the former White Houser national security adviser John Bolton.

In public, Democrats have flatly rejected the prospect of Biden, the son of the 2020 Democratic presidential contender Joe Biden, appearing as a witness in the impeachment trial, but The Post said they're discussing the possibility in private conversations.

The younger Biden's work as a consultant for Burisma, a Ukrainian energy firm, is at the heart of the events that resulted in Trump's impeachment.

John Bolton, then the US national security adviser, standing by President Donald Trump in the Oval Office in 2018. Saul Loeb/Getty Images

Trump dispatched allies such as his personal lawyer Rudy Giuliani to investigate unsubstantiated allegations that Joe Biden helped quash a corruption allegation into Burisma when he was vice president to help his son.

Democrats say the search for a Biden investigation was a smear job intended to damage the elder Biden's presidential candidacy. Republicans say the president was concerned more broadly about corruption in Ukraine.

It was a whistleblower's complaint about a July 25 phone call in which Trump urged Ukraine's president to launch a Biden investigation that ultimately led to Trump's impeachment.

The White House resisted providing any witnesses or evidence to the impeachment inquiry, but Bolton, who left his position in September, has said he would be willing to testify before the Senate impeachment trial if issued a subpoena.

Fiona Hill, a former White House Russia adviser who testified in the impeachment inquiry, said Bolton was highly critical of Giuliani's activities in Ukraine, referring to them as a "drug deal," and observers believe he would be likely to have information damaging to Trump's defense.

Trump's impeachment trial will continue Wednesday as the House impeachment managers begin opening arguments.

Among the key questions for the trial is whether to allow new witnesses to testify, with Democrats needing the support of only four Republicans to win a motion for new witnesses.

"If there are four Republicans who open the conversation about witnesses, then I assume there will be a negotiation that follows — but I can't tell you where it goes," Sen. Dick Durbin, the Democratic whip, told The Post, refusing to rule out the prospect of Biden testifying.