WASHINGTON ― The biggest unanswered question about the impeachment trial of President Donald Trump is whether it will feature any new witness testimony.

But even if Democrats can’t drag in White House officials like acting chief of staff Mick Mulvaney or former national security adviser John Bolton, they already have evidence from someone much better: Donald J. Trump himself, who has provided some of the most incriminating remarks in the impeachment trial.

After an unidentified official blew the whistle on the president last August for trying to coerce a foreign government into helping him win the 2020 election, the White House released a partial transcript of the phone call that sparked the complaint. Republicans contend that it shows Trump cares about corruption; Democrats say that it shows Trump asking the Ukrainian president for a sham investigation into former Vice President Joe Biden.

If there was any doubt about which side is right, Trump has explained exactly what he wanted from Ukraine.

“I would think that, if they were honest about it, they’d start a major investigation into the Bidens,” Trump said outside the White House on Oct. 3 last year. “It’s a very simple answer.”

During the first three days of arguments in front of the U.S. Senate, Democratic House impeachment managers played the clip of Trump’s comment at least four times.

As the president’s legal team assumes control of the proceedings on Saturday, they’ll have to come up with a credible explanation for his comments ― something that Trump’s defenders in Congress have struggled to do so far.

“I can’t imagine being in the position of being President Trump’s attorney, because he says things all the time that are directly contradictory to his own interests, to his defense strategy or to what he said yesterday,” Sen. Chris Coons (D-Del.) told HuffPost this week.

The Oct. 3 clip is just one of several videos Democrats have shown repeatedly at trial. In another, from a June 12 ABC News interview, the president said that if a foreign government offered dirt on an opponent, he’d accept it ― contrary to campaign finance law, which prohibits candidates from accepting anything of value from foreign governments.

“There’s nothing wrong with listening,” Trump said in the video. “If somebody called from a country, Norway, [and said,] ‘We have information on your opponent,’ oh, I think I’d want to hear it.”

In an Oct. 17 video shown at least four times, a reporter explicitly asked Mulvaney to confirm that the Trump administration had withheld military assistance from Ukraine as a “quid pro quo” in return for political favors.

“We do that all the time with foreign policy,” Mulvaney said, adding a moment later: “Get over it.”

Democrats, obviously, think the videos are great stuff, even though they’re not new.

“The video is telling the story very powerfully and the president of the United States and his chief of staff ... basically confessed to the violations that they’re charged with,” Sen. Tom Udall (D-N.M.) said.

Republicans are less impressed. “Old stuff,” said Sen. Pat Toomey (R-Pa.).

But in the months since the videos were recorded, Republicans haven’t come up with a compelling defense of a president openly demanding that a foreign government investigate his political opponent. They have argued instead that the Democrats used a flawed process in the House impeachment inquiry.

Sen. Mike Braun (R-Ind.) acknowledged this week that the Biden request was bad. “I wouldn’t have done it myself, but it’s not impeachable,” Braun told HuffPost.

Rep. Mark Meadows (R-N.C.), one of the House members advising Trump’s trial team, seemed to concede that the president’s statements have not helped his case, but Meadows suggested the public has already factored in those remarks.

“The poor narratives are already out there,” Meadows said. “I don’t see anything that could get any worse.”

Trump’s call for a “major investigation” of Biden by Ukraine points to a fundamental question: Is it OK for the president to ask a foreign government to investigate his political rival?

Many Republicans have avoided giving a yes or no answer. Sen. Josh Hawley (R-Mo.) told HuffPost that, speaking hypothetically, if a president asked a foreign government for electoral assistance, “it would not be a good thing.” But he said that’s not what Trump did on his phone call.

So what did Trump do?

“I think that he asked the Ukrainian president if the government would cooperate in ongoing American investigations, DOJ investigations, which seems fine to me,” Hawley said.

On the call, Trump asked Ukrainian President Volodymyr Zelensky to talk to Rudy Giuliani, Trump’s personal attorney, and to U.S. Attorney General William Barr about investigating both Biden and a conspiracy theory (favored by Trump) that Ukraine, not Russia, meddled in the 2016 U.S. presidential election.

The Department of Justice said in September that it had nothing to do with any investigation into Biden. “The president has not spoken with the attorney general about having Ukraine investigate anything relating to former Vice President Biden or his son,” a DOJ spokesperson said at the time. “The president has not asked the attorney general to contact Ukraine — on this or any other matter.”

Sen. Lindsey Graham (R-S.C.) ― who is probably the president’s biggest Senate defender and someone who has described Trump’s impeachment as a “lynching” ― acknowledged this week that the president might be wrong about a few things.

“The president believes that what happened in the Ukraine with the Bidens was inappropriate. Now, whether or not that will withstand scrutiny, I don’t know,” Graham said, adding that the Ukrainian government was definitely not a bad actor in 2016.