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In the aftermath of the John Bolton bombshell—the news that a draft of his upcoming book recounts the president telling Bolton he was withholding military aid from Ukraine to force that country to launch political investigations to benefit him—some House Democrats are considering demanding Bolton appear before a House committee, perhaps as soon as in the next few days.

Bolton, Trump’s former national security adviser, has become the flashpoint of an impeachment trial fight over whether Democrats can call witnesses who were not questioned during last year’s House inquiry into the Ukraine scandal. During that inquiry, House Democrats asked Bolton to submit to a voluntary interview. He did not agree to that. At the time, Charles Kupperman, a former Bolton deputy in the White House, was seeking a court ruling to determine if he could testify over the objections of the Trump White House. Bolton said he would wait for that case to be decided. Democrats ultimately withdrew their subpoena of Kupperman, and the case was dismissed.

Bolton has been the elusive prize for House Democrats. Trump, his legal team, and his GOP defenders have repeatedly claimed that the House inquiry produced no witness who could say Trump told him or her directly that he was pressuring the Ukrainian president Volodymyr Zelensky, by withholding military assistance, to launch investigations to dig up dirt on Joe Biden and to find material to support an unfounded conspiracy theory that Ukraine, not Russia, had somehow hacked the 2016 election. Gordon Sondland, the European Union ambassador whom Trump used in his effort to muscle Zelensky, testified there was indeed a “quid pro quo” in which Trump would not grant Zelensky a coveted White House visit until Ukraine announced these investigations. But Sondland said that when it came to the military aid being part of Trump’s scheme, for him that was only a “presumption.” White House chief of staff Mick Mulvaney appeared to say at a press briefing last year that there was indeed a quid pro quo that involved the military aid, but he quickly walked back that admission. Mulvaney, too, refused to testify in the House inquiry.

So Bolton could be the witness who provides first-hand confirmation that Trump used the military aid as leverage for his own personal gain.

In response to the news about Bolton’s manuscript, some House Democrats started discussing among themselves whether a House committee should immediately call Bolton to testify. It’s not clear that he would now agree to do so. But Bolton has said publicly that he would testify in the Senate trial if subpoenaed to do so.

The witness issue has not been resolved in the Senate trial. According to the Republican-written rules for the trial approved by the Senate on a party-line vote, a final decision on witnesses will not be made until after each side presents its opening arguments, a process that is still under way. Much attention has focused on whether a handful of Republican senators—including Mitt Romney, Susan Collins, Lisa Murkowski, Lamar Alexander, and Cory Gardner—will join Democrats in pushing for testimony, with Bolton and Mulvaney being two key witnesses sought by the House managers. On Monday morning, Romney said it was “increasingly likely” there would be enough Republican votes to call Bolton as a witness. And Collins put out a statement: “The reports about John Bolton’s books strengthen the case for witnesses and have prompted a number of conversations among my colleagues.”

With the prospect of Bolton appearing as a witness in the trial unresolved—it’s still possible he might not be called—several House Democrats on Monday initiated talks within their caucus about taking the dramatic step of subpoenaing him or once again requesting that he testify before a House committee, according to multiple Democratic sources. Bolton would, under oath, presumably say that Trump informed him at the time that he was leaning on Zelensky by withholding the $391 million in military assistance Ukraine needed for its ongoing war against Russia.

But, according to these same sources, Rep. Adam Schiff (D-Calif.), the chair of the House intelligence committee and the lead House impeachment manager, is not in favor of this, believing that Senate Republicans might somehow use such a move as an argument against calling Bolton as a witness in the impeachment trial. “The onus is clearly on the Senate to call a relevant and willing witness before they vote,” says a Democratic aide working on the impeachment trial.

Several House Democrats raising the idea of calling Bolton to testify in the House note they are sensitive to this, but they say they still intend to raise the matter within the Democratic caucus. They point out that a Bolton appearance in the House—or his refusal to testify—could put pressure on Senate Republicans to call him as a witness in the Senate trial.

The Bolton revelation could turn the Trump impeachment trial upside down. His manuscript contains many relevant disclosures about the Ukraine scandal. He writes that Secretary of State Mike Pompeo believed Rudy Giuliani was trying to get Marie Yovanovitch, the US ambassador to Ukraine, fired perhaps to benefit clients of his other than Trump. He also reportedly notes that he shared with Attorney General William Barr his concerns about Giuliani. According to the New York Times, when Bolton on December 30 submitted a copy of his manuscript to the White House to be reviewed for classified material, this “intensified concerns among some of his advisers that they needed to block Mr. Bolton from testifying.”