President Donald Trump announces proposed rollbacks to the National Environmental Policy Act regulations during an event in Roosevelt Room of the White House in Washington, January 9, 2020.

Neither of the two whistleblowers has been officially identified by name, and neither of the two testified during the House investigation last fall.

The president said he also wanted "the second whistleblower" to testify, referring to a second official who sought legal protections in order to share concerns about Trump's July 25 call with Ukraine's president.

"I'm going to leave it to the Senate, but I'd like to hear from the whistleblower, I'd like to hear from shifty Schiff, I'd like to hear from Hunter Biden and Joe Biden," Trump told reporters at a White House event.

WASHINGTON — President Donald Trump on Thursday said that he would support witnesses testifying in his upcoming Senate impeachment trial, as long as it meant his legal team could summon House Intelligence Committee Chairman Adam Schiff, D-Calif., former Vice President Joe Biden and his son Hunter Biden, and the anonymous whistleblower whose 2019 complaint sparked the probe that ended in Trump's impeachment.

But nearly a dozen current and former diplomats and national security officials did testify, describing Trump's central role in a monthslong effort to pressure Ukraine's government into launching investigations into Trump's political opponents.

Most notably, they said Trump wanted an investigation into former Vice President Joe Biden's son Hunter, who once served on the board of a Ukrainian energy company. Trump also wanted Ukraine to investigate a debunked conspiracy theory that alleged Ukrainian interference in the 2016 U.S. presidential election.

The testimony served as evidence in an investigation that resulted in the House formally voting late last year to approve two separate articles of impeachment against Trump — one on the charge of abusing the powers of the presidency, and another on the charge of obstructing Congress.

Speaking on Thursday, Trump also said he would not object to his former national security advisor, John Bolton, testifying in a Senate trial. But any testimony Bolton gave would be subject to limitations of executive privilege, Trump added.

"I would have no problem [with Bolton testifying] other than we have to protect, we have to be able to protect" executive privilege, Trump said. "People can't go up and say whatever my thoughts are, whatever your thoughts are, about us, countries, views, you don't want that to be out. So we have to protect presidential privilege."

Bolton was present for many of the key moments in U.S.-Ukraine relations over the past year, and if witnesses were to be permitted at a Senate trial, Bolton would likely be at the top of any Democratic witness list.

On Monday, Bolton said publicly that if he were subpoenaed by the Senate, he would be willing to testify in an impeachment trial.

Bolton did not testify in the impeachment investigation last year, after his lawyer threatened to go to court to fight any subpoenas issued by House committees.

As of Thursday, the question of whether to call witnesses in a Senate trial remained a central sticking point between Republicans and Democrats in the runup to Trump's impeachment trial.

Senate Minority Leader Chuck Schumer, D-N.Y., has been insisting that new information that has come to light since December necessitates the calling of new fact witnesses to testify about it.