Bolton, Eisenberg and Ellis are emerging as key figures in House Democrats’ impeachment inquiry, which centers on allegations that President Donald Trump sought to leverage U.S. military aid and a White House meeting with Ukraine’s president as part of a pressure campaign for the eastern European country to investigate Trump’s political rivals.

Bolton’s attorney, Charles Cooper, also represents Charles Kupperman, who served as Bolton’s deputy. Kupperman last week asked a judge to decide whether he must comply with a congressional subpoena for testimony, or defer to the president’s direction that he not appear.

Kupperman’s case is viewed as a proxy for whether Bolton will end up cooperating. Richard Leon, the judge assigned to Kupperman’s case, will set the briefing schedule at a hearing Thursday. Democrats who serve on the three committees leading the inquiry have indicated they are not sure whether Bolton will end up appearing.

“He’s unpredictable,” said Rep. Stephen Lynch (D-Mass.), a senior member of the House Oversight and Reform Committee.

At least two witnesses who have already testified before impeachment investigators have named Eisenberg and Ellis.

Fiona Hill, Trump’s former top Russia aide on the NSC, testified earlier this month that she met with Eisenberg twice at Bolton’s urging after she was unnerved by efforts to involve Trump’s personal attorney Rudy Giuliani in official Ukraine matters. Hill also quoted Bolton as calling Giuliani “a hand grenade who’s going to blow everybody up,” according to a source familiar with her closed-door testimony.

Lt. Col. Alexander Vindman, a senior NSC aide focusing on Ukraine, told investigators on Tuesday that he twice reported his concerns about Trump’s posture toward Ukraine, including Trump’s July 25 phone call with Ukrainian President Volodymyr Zelensky. During that conversation, Trump pressured his Ukrainian counterpart to launch an investigation into former Vice President Joe Biden, and another focused on a debunked conspiracy theory about Russia’s interference in the 2016 election.

“I did not think it was proper to demand that a foreign government investigate a U.S. citizen, and I was worried about the implications for the U.S. government’s support of Ukraine,” Vindman said in his opening statement, which was obtained by POLITICO.

Ellis is a former aide to California Rep. Devin Nunes, the top Republican on the Intelligence Committee and a top ally of the president. Ellis was identified last year in reports as someone who facilitated Nunes’ access to intelligence information that the California lawmakers later shared with Trump regarding the president’s associates swept up in U.S. government surveillance.

Kyle Cheney, Natasha Bertrand and Daniel Lippman contributed to this report.

