WASHINGTON (Reuters) - President Donald Trump goes on trial in the U.S. Senate this week on charges he abused his office and obstructed Congress, with one of the most contentious questions still unresolved - should senators hear from witnesses?

Senate Democrats in the Republican-controlled chamber want four current and former Trump administration officials to testify. Democrats need at least four Republicans to join them to reach a majority of 51 senators to approve the subpoenas.

Many Republicans, however, want a speedy trial of Trump, their party leader, without witness testimony. Trump has at times said he wants to hear from witnesses, albeit not the same ones the Democrats want to testify.

Here are some of the potential witnesses in the trial.

JOHN BOLTON

Bolton is Trump’s former national security adviser. Democrats believe he has first-hand knowledge of key decisions made to pressure Ukraine to investigate Trump’s rival, Democratic presidential contender Joe Biden.

Congressional investigators believe Bolton objected to Trump’s decision to delay $390 million in military aid to Ukraine that is at the center of the impeachment case and could elaborate on that, a Senate aide told Reuters.

Bolton refused to participate in the House of Representatives impeachment investigation, but in a surprise development said on Jan. 6 that he was willing to testify in the Senate trial.

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MICK MULVANEY

Democrats have argued that Mulvaney, Trump’s acting White House chief of staff, was directly involved in withholding aid to Ukraine and has first-hand information to share.

Mulvaney in October acknowledged during a news conference that the White House withheld the money in order to push Ukraine to carry out the investigations sought by Trump, although he later reversed those comments.

Mulvaney was one of several Trump administration officials who defied subpoenas issued by the House when it was investigating Trump.

MICHAEL DUFFEY

Duffey is a political appointee in the White House budget office who oversees national security funds. Duffey ordered the Pentagon to freeze the security funding for Ukraine, according to publicly released government documents.

Duffey also did not cooperate with the House impeachment investigation.

ROBERT BLAIR

Blair is an assistant to Trump and a senior adviser to Mulvaney. Blair was on the line during a July 25 phone call between Trump and Ukrainian President Volodymyr Zelenskiy at the center of the impeachment inquiry. Like Mulvaney and Duffey, Blair defied a House subpoena for his testimony.

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JOE BIDEN AND HUNTER BIDEN

Trump has alleged that when Biden was vice president he tried to have Ukraine’s then-chief prosecutor fired to stop him investigating Burisma, an energy company his son Hunter worked for. This claim has been widely debunked and Trump has offered no evidence to support the accusation.

Trump told reporters on Jan. 9 that he still wanted to hear from Biden and his son during the trial. Biden has accused Trump and his personal lawyer Rudy Giuliani of peddling “false, debunked conspiracy theories.”

Biden said on Dec. 28 that he would comply with a Senate subpoena but that there would be no legal basis for issuing one. He had been criticized for previously suggesting he would defy a Senate subpoena.

THE WHISTLEBLOWER

Trump has said the Senate trial should include testimony from the whistleblower who prompted the impeachment inquiry by lodging a complaint with an internal government watchdog about Trump’s July 25 phone call with Zelenskiy.