Lead House impeachment manager Adam Schiff spelled out why lawmakers should call for John Bolton’s testimony during a senator question-and-answer session on Wednesday: Bolton has the firsthand information that could upend President Donald Trump’s defense.

“If you have any question about [Trump’s actions] at all, you need to hear from his former national security adviser. Don’t wait for the book,” Schiff said. “We think the case is overwhelmingly clear without John Bolton, but if you have any question about it, you can erase all doubt.”

Over and over, as they responded to questions about the facts of the case, the managers urged senators to subpoena Bolton so they could learn directly about his knowledge of a quid pro quo between Trump and the Ukraine government. The House impeachment articles stated that Trump abused his power by conditioning military aid to Ukraine on the announcement of political investigations into the Bidens and alleged 2016 election interference.

Bolton, in a new book manuscript, says Trump explicitly told him about conditioning aid on the political favor, according to a report the New York Times released this week. That report has since spurred fresh interest in hearing from witnesses during the trial, and the impeachment managers on Wednesday emphasized that such testimony could allay some of Republicans’ chief questions about Trump’s motivations.

“All quid pro quos are not the same. Some are legitimate, and some are corrupt. And you don’t have to be a mind reader to figure out which is which. For one thing, you can ask John Bolton,” Schiff said.

Bolton’s revelations and potential testimony are so explosive because they undercut a central part of Trump’s defense argument: Attorneys for the president have argued that there have been no specific witnesses that Trump told directly about the quid pro quo.

Trump’s defense team has argued his behavior doesn’t rise to the level of an impeachable offense, even with Bolton’s account. Lawyers have also suggested that if his account was really that crucial, Democrats should have worked harder to secure witnesses for House testimony.

The managers have explained that Bolton was not deposed during the House inquiry because the president directed administration officials not to testify. They also said they refrained from issuing Bolton a subpoena because of the extensive legal battle that would ensue. (A similar fight over a House subpoena issued to former White House counsel Don McGahn, for example, has been ongoing for months.)

It is worth noting that questions remain about how calling witnesses would work. If Democrats wanted to call Bolton, for example, Republicans have suggested they’d be interested in a one-for-one trade for someone they are interested in, like Hunter Biden. Democrats, however, have chafed at that suggestion, since they don’t see Hunter Biden as having any direct knowledge about Trump’s wrongdoing — the focal point of this trial.

And because of the uncertainty about the actual terms of witness testimony, both sides have been able to offer conflicting visions of what that process would look like.

Take Trump counsel Patrick Philbin’s scenario: “It’s not a question of one witness. The president would have an opportunity to call his witnesses. If the body were to go in that direction, this would have to drag on for months and prevent it from getting its business done.”

But impeachment manager Hakeem Jeffries on Wednesday emphasized that Congress would be able to address witness testimony in an “expeditious fashion,” citing the House’s own inquiry when up to five witnesses were deposed in a week. Jeffries noted, too, that Chief Justice John Roberts could help settle any debate over whether executive privilege could be applied to bar Bolton from speaking, preventing extensive legal battles from playing out.

House managers are making their arguments at a pivotal time. This Friday, the Senate will vote on whether it’s interested in calling more witnesses for the trial or if it’s content to move to a quick acquittal. As of Tuesday, Senate Majority Leader Mitch McConnell had said he didn’t yet have the votes to block witness testimony altogether.

Managers, in their remarks on Wednesday, attempted to appeal to the handful of lawmakers who had yet to make up their minds. “In every single [impeachment] trial, there were witnesses. Why should the president be treated differently?” Jeffries said.